It's nice to see that you guys are still interested in this subject.
After 2,5 years my parents system is still running smoothly. (Although, it was a little bit faster 2,5 years ago)
In time I have learned a lot about system speed, stability and security.
This is a very good written topic, I would like to add some information:
By following these tweaks, we have to keep in mind that we take a great risc to cause system instability or affect it's normal behaviour. (ex. disabling the wrong Windows Services).
For those who are not technical I would suggest to install as minimal software as possible. Install antivirus,firewall,anti-spyware tools. Defragment and run the Disk Cleanup regularly. Keep your system up-to-date and make regular backups. Before trying these tweaks, be sure to know what you are doing.
So here are some basic tips:
Have a Windows/Linux/SystemRescueCD LiveCD near you.
Be prepared to fail: Copy your Windows Bootloader/GRUB to a floppy/USB Stick (make sure you have prepared your system to boot from floppy/USB).
Install an "untweaked" Windows version on a second partition and make it ready for immediate use at all times (MS Office, WLM, etc).
It might be useful to move the "Documents and Settings" folder to another partition (I haven't done this yet):
How can I move Documents and Settings to another partition? - Bink.nu
In order to understand which Windows Services are safe to disable, you could read more on the following link:
Windows XP Home and Professional Service Pack 2 Service Configurations by Black Viper
It takes time to learn which services could be disabled, so there is also a "one-click" solution available, check out the following link:
Windows XP Service Pack 2 Services Registry Files and Information
We only have covered the Windows Services. We also have other services/startup programs. Find and disable them if you don't need them. Start -> Run -> MSConfig
10. Install, run, "Do a system scan and save a logfile", copy the content from notepad and paste it to a HijackThis Log File Analysis.
You could check out which entries are safe, and delete the entries which are not.
HijackThis Log File Analysis: HijackThis Logfileauswertung
Through time, I have also learned that installing less applications/software on your computer is much better to keep your system running smoothly (It leaves (temporary) files, leaves registry keys even when you have uninstalled it)
Don't install software unless it's absolutely neccessary. It might be an option to install VMWare/VirtualPC and try out software in a virtualized environment.
Try to run as much as portable applications as possible in order to prevent that registry keys/temporary files are stored on your Windows partition. For a list of portable programs check out: Applications | PortableApps.com - Portable software for USB drives
Make sure that you install/store as little as possible on your windows partition.
Make a backup of your Windows partition after you have completly installed and configured it correctly. If your system becomes slow, you could simply restore an old backup. If you use the portable apps and run it from another partition your software is still up-to-date without having to reinstall it
Don't work from the Administrator account, but create a limited account. This could be used for security reasons but also a limited account is not able to write to a lot of areas on the system, which prevents your apps mess up your system globally.
If you like the Windows Theme, but would like to disable the Visual Effects, try the following: Control Panel -> System Properties -> (Advanced) -> Performance (Settings) -> Visual Effects -> Disable everything except "Use drop shadows for icons labels on the desktop" and "Use visual styles on windows and buttons".
Some of these temporarily files are not removed by ccleaner/cleanmgr.
For more information about temporary files check out:
Disk Cleanup: deleting unnecessary files.
System Volume Information
It would be nice if someone would like to write a batch file which cleans up the above temporary files or run "ccleaner.exe /auto", cleanmgr /sagerun:1, chkdsk, defrag, etc.
Edit: You could also defragment your Windows partition effectively if you run Windows from a LiveCD or from a different partition.