Here is an error that is somewhat puzzling. But the solution was simple enough.
When converting an existing Microsoft Virtual PC instance to WMWare, I got this error:
Unable to load the source virtual machine or image. The file might be corrupt, or of an unsupported format.
The Log Info link directed me to C:\Windows\Temp\vmware-temp\
Opened vmware-converter-0.log file to find this:
[#3] [2009-04-27 19:28:38.626 'App' 5696 verbose] [,0] DISKLIB-LIB : Failed to open 'D:\VPC\MOSSDEV\CommonStore.vhd' with flags 0x15 (The system cannot find the file specified).
The VPC to be converted references a VHD file in a different folder than where the .VMC file is. Apparently the wizard is looking for .VHD files in the same folder, regardless of the actual setting...
A quick edit of the VPC settings (remove or move the VHD file) and all is well.