Ok, the most likely cause of the problem, assuming the computer is powerful enough to run whatever it is you are running, is fragmentation. How long is it since you have defragmented your hard drive?
Click on Start, and go to All Programs. Then from the list that comes up, click on Accessories, and then System tools from the new list that comes up. Then click on Disk Defragmenter.
A window will pop up. Click on the C:\ drive on the list, and then click Analyse. This will analyze your hard disk and tell you if you need to defragment. It will also show you a graph of the fragmentation of your hard disk. Basically, more red means more fragmentation.
If it says you need to defragment, then click defragment. THIS WILL TAKE A LONG, LOOONG TIME (but your computer should run much faster afterwards).
Basically, fragmentation is how files are split over your hard drive. As you use your computer, files are constantly being created, moved and deleted. The disk isn't necessarily filled up from top-to-bottom, and so through constant use the files get split throughout the disk. Defragmentation puts these parts of the files back together, so it takes a lot less time to access, thus speeding up the computer.
You should defragment your computer regularly (the more often you do it, the less time it will take. If you haven't defragmented at all yet, then it's gonna take a hell of a long time to do THIS TIME).