I have tried to organize this list in order by those tips which will give the most boost to your XP/Vista/Windows 7 preformance. However, your results may vary. In rare cases (and as a last resort!) it may worth backing up all data files you wish to keep and installing a fresh copy of XP/Vista/Windows 7. Caution! Be sure you have all the original applications CDs/DVDs - you will need to install them after the new XP/Vista/Windows 7 install. If you have never done a fresh install of your operating system before and you have many applications, be warned that this is a very tedious process and may require several hours to complete. I cannot promise that all the tips listed here will give you noticeable preformance enhancements, but they have been ones I have personally used and so far as my individual testimony is concerned, they have been effective. If you are a relative computer novice you may want to stop at the first three steps. And if you use Symantec/Norton Security tools (or other securiy product) you may elect to not install the apps I suggest in step 1. My experience with Symantec/Norton over the years has been an unhappy one. They consume significant system resources and sometimes have caused major system problems. I do not recommend them to anyone. However, if you are happy with them, my best wishes!

1. Eliminate spyware/viruses

Here are the anti-virus/antispyware products I use: all FREE!
First of all: a little background on spyware and computer viruses from Wikipedia. Secondly: How many anti-virus/antispyware products should you install?

For all Windows users connected to the Internet one anti-virus and at least one anti-spyware product are recommended. In general you should not try to install multiple anti-virus programs as they will interfere with one another (and often will not even install if they detect another anti-virus installation). Multiple Anti-spyware programs do not pose the same problems.

For users with older computers (slower, single core processors and 512 meg or less RAM) I recommend the following:

Panda Cloud Antivirus Free Edition
Sets a new technological standard for virus/spyware/malware protection. See the PC Mag review. I now recommend Cloud over all other anti-virus products. Requires free and easy email registration.

Avast Home EditionI have tested Avast on four of my computers (testing under XP, Vista, and Windows 7) and am pleased with the results.

For users with one gig or more of RAM and multi-core processors: AVG Anti-Virus/Anti-Spyware Free Edition Version 9

Anti-spyware:

IObit Security 360

"IObit Security 360 is an advanced malware & spyware removal utility that detects, removes the deepest infections, and protects your PC from various of potential spyware, adware, trojans, keyloggers, bots, worms, and hijackers."

Malwarebytes' Anti-Malware "Can detect and remove malware that even the most well known anti-virus and anti-malware applications fail to detect."

SUPERAntiSpyware FREE Edition "Can detect and remove malware that even the most well known anti-virus and anti-malware applications fail to detect."

Spyware Terminator Fast spyware scanning 100%, real-time protection, HIPS protection, Antivirus protection. All in one free package! Does excellent analysis of running and installed programs to show you potential threats.

If you do get an infection here are some useful links:

How to remove malware for free: YouTube Video

REMOVE-MALWARE.COM

Here is a review of Six free antivirus programs made for your Windows 7 system

Included are reviews of Panda, Avira, Avast, Microsoft Security Essentials, AVG, Comodo.

2. Clean your registry

Advanced SystemCare Free
"Advanced SystemCare Free has a one-click approach to helping protect, repair, clean, and optimize your PC. With over 20,000,000 downloads since 2006, this fantastic, award-winning, free program is a "must-have" tool for your computer,"

Glary Utilities "Offers numerous powerful and easy-to-use system tools and utilities to fix, speed up, maintain and protect your PC. It allows you to clean common system junk files, as well as invalid registry entries and Internet traces. You can manage and delete browser add-ons, analyze disk space usage and find duplicate files. You can also view and manage installed shell extensions, encrypt your files from unauthorized access and use, split large files into smaller manageable files and then rejoin them. Furthermore, Glary Utilities includes the options to optimize memory, find, fix, or remove broken Windows shortcuts, manage the programs that start at Windows startup and uninstall software. Other features include secure file deletion, an Empty Folder finder and more." Another good dedicated registry cleaner: RegScrubXP. Caution! Not all registry cleaners work alike - some are overly aggressive and may cause problems. I have tested Advanced SystemCare, Glary, and RegScrubXP many times without a hitch.

3. Remove unused programs

Control Panel/Add/Remove Programs (Or BETTER, use Advanced SystemCare or Glary Utilities). Re-run Advanced SystemCare/Clary Utilities/Registry Cleaner afterwards - many programs do not properly clean up their registry entries during their uninstall process leaving junk behind.

4. Eliminate useless services:

Start/run/services.msc

For XP see: Windows XP Service Pack 3 Service Configurations

For Vista see: Windows Vista Service Pack 2 Service Configurations

For Windows 7 see: Windows 7 Service Configurations

5. Prune startup programs/System Tray apps

Use Start/Run/msconfig to stop useless startup programs and System Tay apps. (Or BETTER, use Advanced SystemCare or Glary Utilities/Startup Manager).

6. Defrag HD

Start/programs/Accessories/System Tools/Disk Defragmenter

If you use machine heavily (create/manipulate many large files routinely) install IObit SmartDefrag

Free, faster, more thorough than Windows' built-in defragger.

7. Eliminate unused desktop icons

Every time the desktop loads it must present the icons for the various applications, data files, and shortcuts stored on the desktop. If you have many icons it may take sometime for the icons to be read. If you want to store data files on the desktop at least create folders for the files and move teh data file to the folders. Eliminate icon representing shortcuts to to apps you use infrequently. After all you can always find them in the Programs Menu.

8. Turn off system sounds

Control Panel/Sounds and Audio devices/Sounds/No Sounds

9. Set spartan folder options (you can always select other views when necessary).

Use Details view

Turn off unnecessary attributes

Tools/Folder options/View/Uncheck the Automatically search for network folders and printers check box

Tools/Folder options/View/Apply to All folders

10. Turn off some Visual Effects

My Computer/Properties/Advanced/Performance
Leave only the following checked:

Show shadows under menus

Show shadows under mouse pointer

Show translucent selection rectangle

Smooth edges of screen fonts

Use drop shadows for icons labels on the desktop

Use visual styles on windows and buttons

(Bonus tip) 11. Don't use Windows Media Player, rather install VideoLAN

"A free cross-platform media player, supports a large number of multimedia formats, without the need for additional codecs. It can also be used as a streaming server, with extended features (video on demand, on the fly transcoding, ...)" Highly recommended.

Some good XP optimization pages

Optimize XP

The Complete Guide To Optimising Windows XP

The TweakGuides Tweaking Companion (for XP and Vista)

Windows XP Super Tweaks

XP Myths

Some good Vista optimization pages

Windows Vista Super Tweaks

Some good Windows 7 optimization pages

Windows 7 Super Tweaks

A free Windows 7 book (360+ pages pdf)

Windows 7 - The Pocket Guide
(requires free registration)

Free from Microsoft:

Windows 7 Product Guide (140 pages pdf)
Profusely illustrated in color