Maybe this will help you.
I did find: http://stackoverflow.com/questions/38952/unable-to-determine-guest-operating-system-vmware-error
This is a fairly generic error from VMware Converter so I would try the following:
Step 1. Make sure you are running the latest version of VMware Converter. Updates seem to come pretty often for this tool.
Step 2. Check the VMware Converter log file. More often than not you will find the source of your problem here.
Here's another possible workaround:
