Never use defrag ! We use it for security
purpose when we went to make sure that nobody
can recover what was on a disc by randomizing
the data !

If your windows start behaving realy strangely
and you are sure it's not a virus, backup
the data, trash the whole partition and reinstall
windows. Only way to get clean library and a
healthy registery database.

If after that, random files keep being corrupted
regularly, that's probably an hardware problem :
either the disk or the controler. (nearly as
good as defrag to randomize data on the disk)

Cheers,
Yann Stettler