Have you perused your task manager to see if something's eating up your memory? If something is using an outstanding amount of memory you could have an optimization problem or a virus.

You can also run the msconfig utility and disable select startup programs and services, and that will generally speed things up - just be sure you know exactly what you're turning off.

If your case is particularly dusty, cleaning it out with compressed air can improve performance as well.

The last bastion I can immediately think of is a bad RAM stick. After trying the previous things, run the Windows Memtest utility and see if it spits an error at you. If you've got a bad RAM stick, you'll have to find it and remove/replace it.