Lost Product key question for Vista

I've tried this with a retail version and worked ok never tried it on OEM. So before I screw this Dell up even further...

Someone brought me a dell laptop with vista ultimate (OEM) that they are having numerous dll problems with. They cannot find the OEM disk or product key that came with the computer.
The retail version I have has already been registered.
Can I reinstall the retail version onto the dell using the dell oem product key, that came with the computer.
Which brings me to my next question is there a way you can bring up a product key on the computer.

I figured I ask the question before I sent this person out to buy another copy of Ultimate.