You most probably have too many programs running on your machine. Try uninstalling all the programs that you don’t need any more, remove the programs that start every time Windows starts: Start -> Run, type msconfig and press “OK” -> click on the “Start-up” tab and un-check the programs that you don’t want to start every time you boot up.

Sometimes Norton and Mcafee slow the system down too. You should consider un-installing them and installing anti-virus that doesn’t use so much resources.

Try defragmenting your hard drive(s):
Start -> All programs -> Accessories -> System Tools -> Disk Defragmenter

If you’re running Windows XP right click on “My Computer” -> Properties -> Advanced -> Adjust for best performance.