Jim,
If you search the archives for this forum or Google, you'll see that numerous people have experienced this since XP SP3 was released. It apparently contained some OLE and security "fixes" that have affected Framers who import Visio files and other info as OLE objects.
One solution is to back off SP3; a workaround is to import any problem files as referenced graphics rather than as objects.
If you don't have OLE objects or don't have SP3.... it's a different ball game, and I'd suspect some kind of file corruption. Have you tried saving out as a MIF file and reopening that, to clean things up?
Art