Try to install the USB drivers that come with the Android SDK?
(I don't think you need the HTC sync stuff for that).
Depending on what ROM you are running, or if it was made by some paranoid network provider, you might not be able to use the phone as a modem (tethering)... I've heard that some network providers disable the tethering feature...
With CM ROM, I've had no problems tethering over USB in: Windows Vista, Windows 7, Ubuntu 10.4 (Windows XP didn't work at all).
You have to enable the tethering feature first on the phone before it will work (I'm not talking about installing some app from the market, I'm talking about the built-in setting: settings-->wireless&networks-->tethering).
Have you tried mounting your SD card to access it as storage device over USB?
If neither tethering or USB mount wants to pick up when you plug the cable in, I'm guessing your cable is bust or your phone...
Have you tried the same cable on another PC?
Are you using some dodgy USB add-in card or USB hub on your PC? Did Windows 7 install the correct drivers for your USB controller on your motherboard? (do any other USB devices work on your PC?)