Yeah, of course Google was the first thing I checked, but didn't find anything that looked like a fix - just page after page of people saying they had the problem.

I did run chkdsk. It found a zillion cross-linked files and repaired a bad sector. All the cross linked files were music files; I wonder if my iTunes sync program (iSyncr) is the culprit. But now, all the cross-linked files show up twice in the music player, so I'm just reformatting and resyncing.