Check your free disk space. Windows permormance starts to drop when it's less than 15%. Do you defrag regularly?
How much RAM is installed?
Go to Start > Run > msconfig > Startup tab and post back with the full list of programs. There might be some that run at startup and continue running in the background that could be disabled or uninstalled to increase your computer's performance.
Have you checked inside the case for any faulty or dirty fans? If your computer's temperature goes too high it can cause slowdown and other problems.
SensorsView or
SpeedFan will show your system temperatures and voltages.