Just registered to report that I have similar issues:
WinXP SP3, Word 2003, happens only after the latest update.
When I open a document stored on a *network share* in Word 2003 by double-clicking it in Explorer, Word open some *.tmp file instead. Maybe because I have the option to temporarily store network files locally turned on.
When I turn this Word option off, the file opens, but saving it gives me an error message saying that Word is unable to delete its latest backup copy.
When I do nothing but simply open a file, close Word, and open that file again, Word says that the file is in use and cannot be opened.
When I go to Avast's prefs for the realtime scanner and turn the option to scan documents on opening off, most of these issues no longer occur. (However, Word still leaves a ~$... file in the document's folder when I open a file and then close Word.)
I /have/ to work with an XP installation and Word 2003 every day, so my only choice ATM was to replace Avast. After that, all of the above issues were gone.