Recently my pc has slowed down considerably when closing files, or when I switch between open applications.
A couple of months ago, when this happened, I ran virus checks, scandisked and did other things to clean up the hard disk, as suggested in a couple of books e.g. Windows 98 for dummies. (I think I also tried to use Disk Defragmenter but I seem to remember that it couldn't manage to complete it.)
Things improved, not dramatically but noticeably, but I have noticed a sudden and fresh decline. My antivirus is updated regularly and I use Zone Alarm.
Where I get a message, it usually has something to do with memory. My pc's just failed to save a file because of insufficient memory.