>You were certainly right on that suggestion. The event logs showed many errors, including quite a few disk i/o errors. I will never know exactly how or when the problem began but I still suspect it was associated with the backup software. I ended up purchasing a new HD and restoring from an older backup, then copying only the newer files that were truly essential. The procedure was probably overkill, and took a week to fix (formated the new disk at least 3 times to get the restore done correctly), but everything seems to be working now. The down side is the lost productivity and stress; the up side is I have lots of disk space now :-)

Glad you got it sorted.

These days (mechanical) hard drives are incredibly cheap. If you rely heavily on a particular machine it's a good idea to have at least 2 hard drives in a mirrored (RAID1) array. Everything you do is simultaneously copied to 2 separate drives, not just one. If one fails, you're still OK until you can get a replacement drive and return to full redundancy. These days most new desktop motherboards support hardware RAID directly, as do the majority of boards less than, say, 3 or 4 years old.

It's worked well for me; I've had 3 complete drive failures of WD Caviar Black 1TB drives since Feb. 2010, but zero data loss from any of those failures.