I'm having the same problem, only I get error message DEFRAG00205, which indicates that I need to empty the applog folder. But that doesn't solve the problem.

It took me 8 hours to run a thorough ScanDisk, but then Defrag stops at ~ 10% and says to run ScanDisk.

I'm in safe mode, ~4 GB used on a 20 GB hard drive. I've physically cleaned the innerds of the box with compressed air. I'm currently throwing away digital pictures that I can live without. My virus checker doesn't detect any viruses.