I had the same problem starting around 3/15/09. In my case, only Office 2007 files stored on a network drive were affected. I am running Vista Home Premium.
I could open an Office 2007 file from the network drive if it hadn't been touched since before the McAfee update. However, I would get a "file in use" error when I tried to save the file back to the network drive. Once the file was opened and closed, it could no longer be opened, moved, or copied because of a "file in use" error.
These issues apply only to MS Office 2007 files - all others work fine.
After much frustration and experimenting, I found a workaround that seems to fix:
Disable real-time scanning of network drives
It appears that an update issued sometime around 3/15/09 does bad things to Office 2007 files. It looks like a temp file is being created by something other than Office (McAfee?) when the file is opened and not being released/deleted. It's not possible to delete the temp file, even as an administrator. The temp filenames are of the form: 2F4ABDD.tmp
Hope this helps someone.