Parallels keeps setting Parallels as preferred app for Office files

Since upgrading from Windows 2000 to WIndows XP on my Parallels volume, Mac OS X 10.5.8 keeps resetting the default application for Microsoft Office files to Internet Explorer [sic, there isn't even a Office on that Windows volume] via Parallels and even selecting a file, setting it to the appropriate Microsoft Office app and selecting change all does not help.

It keeps getting reset. Somehow, Parallels 4.0 keeps telling that it is the default app for .doc and .xls files. How do I fix this?

G