Are you running VCDS-Lite 1.1? Because that is the current version of VCDS-Lite and it is compatible with 64 bit Win 7 machines.
The product is no longer called VAG-COM as VW forced us to change the name. We change it to VCDS (which stands for VAG-COM Diagnostic System). If you are trying to run VAG-COM 409, it will not work with Win 7 or a 64 bit machine. It was released in 2004, designed for XP and 32 bit machines. Further, the license we have sent will not work with 409 - only VCDS-Lite.
Re-installing on the same machine should generate the same serial information - but, if there is a virus or something running, it may not. Also, eliminate as many programs as possible, especially anit-virus programs, as other programs may be interfering with VCDS-Lite (not a normal occurrence but we have some reports that some apps will mess with VCDS-Lite).
Go to the About screen and look at the serial number that VCDS-Lite generated. Compare it to the one you sent to buy your license. If they are the same, then the license we have sent should be fine. If they are not the same, you are going to have to contact our tech support for further
help.