If I remember right, there's a registry problem that sometimes causes this. Do you have iTunes installed? Try uninstalling it, and see if your drives appear then. I found this out after a while of searching while working on a client's Vista machine. I'll try to find the webpage I used....but it was a while ago lol.
Edit: did a quick google and Apple has a troubleshoot page...I believe this is what I had to do:
Optical drives not recognized in Windows Vista after installing iTunes 7