if you can get into device manager ( in XP its- right click on my computer > properties > hardware tab > device manager) find where the cable is listed under
universal serial bus controllers', right click on it and hit properties. you should be able to change the COM port number in one of the tabs. it needs to be in COM1 2 3 or 4 for VAGCOM to recognize it. whatever number you set it to, select that number in VAG setup.

if you need more information google "change COM port number"

 

on vista it's windows > right click computer > properties > select device manager on the top left of the screen.

 

I personally have used this set of drivers in Vista no problem. http://www.glkinst.com/cmetersoftwar...0Certified.zip

In Vista to change your COM port number open the start menu type devmgmt.msc and open the section labeled ports (com & lpt). right click on the one you are using currently as your cable, click properties, go to the port settings tab, click advanced then change the port number to 1-4 if any are open, or I use port 4 if they are all in use. Click Ok on all the open windows and go back into vagcom and try it out.