#1 What interface hardware are you using?
#2 What version of VCDS/VAG-COM are you using?
#3 Did you read
the notes regarding Vista and that only versions 704 and newer support this operating system?
#4 What vehicle are you trying to work on?
#5 What does VCDS/VAG-COM > Options > Test say?
#6 Is the interface plugged in the car while testing?