File xxx.pvs seems to be not a valid parallels virtual machine configuration file

Hello,
I'm evaluating Parallels Workstation on Windows 2000. I seem to have overcome the occasional annoyances I have had with the software except this message box that pops up about 50% of the time for no apparent reason...

File vanguard_x.y.pvs seems to be not a valid parallels virtual machine configuration file

(x or y are incremented each time I eventually add significant functionality to the guest OS (also w2k), eg an app installation)

When it occurs, it takes 5-10 mins for the problem to go away, irrespective of whether I copy across a replacement PVS file (yes, I keep an archive of successful builds).

Has anyone else seen this?

Thanks for any light you can shed on this.