I've had a similar problem with a friends HP laptop/F.A.S.T. combo. The first thing you need to do is make sure you have the C-Com set to the correct serial port. Usually those Belkin things will choose something like COM3 or COM4. The best way to find out is to go to Start -> Settings -> Control Panel -> System. Once the system properties window appears, choose Hardware, then Device Manager. Scroll down until you see Ports (COM & LPT), and click the little plus next to it. You should see a listing of all of the available COM ports on the computer, and one of them should give you an indication that it's for the Belkin (If it's not there, then it's probably not plugged in or installed correctly). Now that you know the correct COM port number, go back to C-Com and change the serial port to whichever matches the Belkin and give it a shot. If everything matches, you might have a bad USB-to-serial adapter.
The problem we have more than anything else is sometimes we can't get it to start logging, and every once in a while we can't get it to communicate with the ECM.
Hope this helps some.