I just wanted to add a bit to this thread, and say thanks to all who contributed.
Just got my Vag com cable working tonight having been through all the problems above.
The following might help to save future members a bit of time:
1. Com port 2 seemed to be the only one that worked for me despite others being free.
2. Check what com port your internal modem is on - vag com will talk to it, and that'll throw you right off the scent
3. When setting up the com port, go into device manager, then in the console tree click on COM&LPT. (The cable will also be listed under USBs at the bottom of the tree, but you can't change the com port in this section).
4. My laptop wouldn't even recognise the cable until I hooked it up to the car - this is contrary to the instructions from Ross-tech.
Now I just need to decifer the codes...it's most unusual being able to diagnose faults without having to get covered in old oil!
All the best,
Beef