Many people have problems running ScanDisk and/or Defrag, with the program running 10% or 30% and then stalling. Often with a message that Windows is trying to write to the drive. To run ScanDisk/Defrag successfully:
(If you use Windows 95, restart your system, when you see the Starting Windows 95... text on your screen, quickly press the F8 key. From the menu that follows, choose Safe mode, and run ScanDisk & Defrag. When ready, reboot.)
PS: My daughters computer had the same problem and the above solution worked. You may also consider installing an additional hard drive if you have an available drive bay. 30g drives are pretty cheap these days and access speeds for HDs have improved over the past few years. I'm not sure how old your system is, but you may find that a new HD improves the overall performance of the system and increases your available storage space at the same time. All it takes is a screwdriver, a few dollars, and an afternoon. Good luck!