I was looking for a solution to the same problem earlier today. After
trying every suggestion and more, it turned out my problem had
something to do with the file being in a network folder that is
synchronised to a local drive using the Offline Files feature in
Windows XP. Copied the file to a local drive and opened it from there,
and the problem was gone. I was connected to the network and working
with the file from the network folder when the problem occurred. Don't
know if the Offline Files feature had anything to do with it, but it
must have been something to do with the file being on the network.

I suspect this problem could have a range of causes.

Cheers, Barry Slaytor