Cubase Studio 5 is ancient! It was also notorious for being very badly cracked. However - I assume Guitar rig 4 (again, old) works in standalone, and Cubase works fine.
This might be an issue with 32 bit compatibility if your windows is running 64 bit? Firstly though. has it NEVER worked, or just STOPPED working. Solutions will be very different.
Does it appear to be working, but just not produce sound, or does Cubase crash or freeze when you try to use it. We need more info. 9 year old Cubase really is severely in need of an update. Have you done any updates since it worked and now it doesn't.