I have two harddrives on my computer and if you do as well, this tweak can get you some really good performance tweaks...
1) Format the second drive (non-OS drive).
2) Set your paging file to None. Reboot.
3) Set your paging file to the second drive and for 1GB min/max. Reboot.
Now, run a defrag and make sure you have a paging file on the second drive and that it's at the beginning of the drive and one continuous block.
This has the benefit of speeding up your machine because both the paging file and applications can be accessed simultaneously.
Turn Off Automatic Updates. Only enable for updating, then disable again. This has to be done through the services program, not through the Automatic Updates dialog.
Turn Off System Restore unless you do a lot of updates, and/or don't consider yourself savvy. In many cases a system restore will save your a$$ but in many more, it will screw things up worse than before.
Visit this site and read about the services installed on your setup. Disable those you don't need or pose security risks...
http://smallvoid.com/tweak/index.html
This site has a lot of great software that can aid in fixing, tweaking your system...
http://www.sysinternals.com/
Disable anything you don't need or use in the Startup folder. Start/Programs/Startup.
Turn off unneeded features in Control Panel/System/Advanced/Performance.
Disable desktop wallpaper and disable themes if you don't care about them.
If you use IE, make sure to \"Delete Files\" often and if you have broadband, set the max size of the Temporary Internet Files folder to 1MB(smallest size).
I don't know if this impacts system performance but I try to keep my My Documents folder fairly clean. In your User profiles, it will show how much space is used by your user and this is effected by how many files are in the My Doc's folder as well.
Defrag...Defrag...Defrag. Don't run just once. Keep runnning until you don't see a noticeable difference between Before/After.
Disable, Remove System Tray Apps that you don't need.
If you feel really savvy, Disable your Anti-Virus. There are plenty of sites like
http://www.antivirus.com/ that you can run once a week or so.
Get Windows Updates frequently. Once a month or so.
Update your drivers frequently.
There's more, but I got to go to work. Have fun!