Parallels 5.0 - XP VM tries to install Parallels Tools each restart - fails

For a while now, every time I restart a particular Windows XP virtual machine on my Parallels 5 system it asks to install Parallels Tools - even though these are installed and working properly.

Now following an upgrade (to 5.0.9376) it continues to ask to install the tools, but the tools themselves no longer work, and reinstalling them doesn't seem to do any good.

This is very frustrating - as I need the tools to allow me to put Parallels into Coherence mode so I can put windows on both the screens attached to the computer.

Can anyone suggest what I could / should do to a) get tools working at all, and b) stop it trying to reinstall every time I restart the XP machine (I think the two fixes are related...).

Many thanks in advance for any help that can be offered.

 

Does anyone have any ideas at all on this?