I've had a similar issues on vista with WMDC where I couldn't get my diamond to mount. I read some troubleshooting guide and they suggest going into device manager and uninstalling/reinstalling the associated usb/mobile device. Another thing I had to do once was delete my partnership with the PC.
This is one of the guides i was looking at:
http://www.pocketpcfaq.com/faqs/acti...shoot-as4x.htm
Is your phone bricked where it can't boot into the phone mode or does that part still work? If you can get to running phone mode, you might want to try changing your USB-To-PC settings?
If you have an HTC Diamond, maybe you could re-flash via Internal Storage?
Or as a long shot, maybe you could try to find someone on this forum in your city that might be able to flash the phone for you. Eliminate the possibility of a PC/Cable/Software problem by using a known, working setup.
Good luck.