I have a standing answer.
http://reviews.cnet.com/5208-6141-0.html?forumID=8&threadID=62632&messageID=743043 notes this error and solutions.
If these don't work, tell more.
Bob
When I try to defragment my C drive,the status bar alternates between, "checking file allocation tables" and "checking folders", and cannot progress past that point. After a few minutes, a message pops up that says,"scan disk has restarted ten times because windows or another program has been writing to this drive.Quiting some running programs may enable scan disk to finish sooner.I had no programs open. Any ideas?

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