Best way would be to use to restore CD (if you have it) for your computer. That basically whipes off everything and takes it back to how it came out of the box.
You could also trying cleaning out the computer (cookies, temp files, etc) by doing a simple START --> PROGRAMS --> ACCESSORIES --> SYSTEM TOOLS --> DISK CLEANUP, if you dont have the cds.
You can also contact the manufacturer giving them your make and model computer and they will send you the restore CD for a relatively modest price -- @35 bucks.
Now, if you have your own version of windows without a restore CD, then you simply format the drive, and reinstall the entire operating system.
TIP -- If youve updated your drivers, or installed hardware you will save yourself alot of time by getting a program that backs up your drivers, so that you dont have to reinstall the drivers once you reinstall the operating system. Theres alot of shareware programs that backup drivers. You can use it once and that will save lots of time.
If you have win XP then
You could also do a Windows System Restore, START --> PROGRAMS --> ACCESSORIES --> System restore and go back to the very first date thats bolded on the calendar. Which will return your system settings back to the way they were on whatever date you had them.
This with your system clean up should do you just fine.
Hope it helps