Either Windows 7 or XP should work equally well.
With the OpenPort 2.0 connected to just your laptop and not the OBD2 port, do you see the 3 LEDs on the OpenPort cycle regularly through the 3 colors? If not, and I suspect this might be the case since you're seeing 'no vehicle interface', then I would first try a different USB cable. It's just a standard mini USB cable like you'd use for most digital cameras, some phones, some GPS, etc.
If the cable is definitely okay but you still see 'no vehicle interface' and the lights aren't cycling, then take a look at the USB carefully. If you look at it from underneath you'll see a seem that if it's spread apart it might mean that the unit has taken a hard hit and the USB connector is deformed. Did you buy this OpenPort 2.0 new?