You never answered the question about how full your hard drive is. If it is
very full, then move some data off it, uninstall unused programs, do
maintenance, etc. to free up space. Defragmenter needs at least 15% of free
space to work.
However, it sounds more to me like your hard drive is failing. I would
definitely back up your data in any case and then test the drive.
Test the hard drive with a diagnostic utility downloaded from the drive
mftr.'s website. You will create a bootable CD with the file you download.
You will need third-party burning software to do this such as Roxio, Nero,
or the free CDBurnerXP Pro. Burn as an image, not as data.
http://www.cdburnerxp.se/
Boot with the CD you made and do a thorough test of the drive. If it fails
any physical tests, replace it.
Malke