Try another download of the upgrade file, the one you have might have been corrupted in transfer. As I was juggling disks this weekend between a MkI and a MkII I had the same sort of thing happen to me. After trying some other stuff I decided to retry the upgrade with a freshly downloaded file - after that everything worked.

I think a published MD5sum of the upgrade files would be a good idea...

Here are the MD5sums of the files as they sit on my computer (only actually used/tried the developer verison though):

car1-consumer-v2.00-beta3.upgrade: 7c9dbe1376e3ef9a23f788ea6eef4b18

car1-developer-v2.00-beta3.upgrade: 67020366f61544c74c0ec939e99cb132

md5sum.exe for Windows is available for instance here

/Michael