Actually, many people forget these steps when their computer is slow.
1. Defragment your disk, especially C drive. You can google it for ways to do it.
Note: don't use third-party programs to defrag it. Use windows disk defragment tool. You can find it in the start menu.
2. Check your disk for errors. If it was a laptop, it may have the problem, caused by moving your computer and overheat. Find it in the properties of C drive if windows xp, and start menu (maybe in system tools) in win vista and 7.
You may google it for ways to do it.
Hope that it works!