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 :-)

Thanks for your help!
Lynda