Is this something that just started occurring or has it been gradually coming on? If it just started, has there been a change made to the computer such as a new program installed or some settings changed? If that is the case then you might try uninstalling the program you installed or putting the settings back to where they were. You could also use System Restore to take your computer back to a restore point prior to the changes you made. If this has been coming on gradually then I would run Disk Cleanup and then defragment your hard drive as Pete said above. If neither of those help then it might be time to move everything you want to save off of the computer, blow the hard drive out, and reload your OS.

Hope this helps!
Bill