Because a messed up registry makes it slow the available hard drive space is essential. When uninstalling programs under Windows XP it's important to do it in the add/remove programs dialog and never delete any uninstall files.

If the program is used for many purposes it's very important to clean up the computer from spyware regularly since spyware can slow down the computer drastically.

You can also do indirect things to make it faster like removing unnecassary programs from the startup sequence. The reason why this is so important is not for the memory allocation, it's because of the disk activity! With less memory available on the disk, low disk speed and high disc activity, the system slows down drastically!

I still want to tell you that performance is best managed with the focus on hard drives.