When I upgraded to LT 2008, about 4 weeks ago, I experienced some "teething troubles" with Vista.
Not having found a solution here on AUGI, I searched over on the Autodesk Discussion Groups too.
Okay the source of my problem was perhaps my own carelessness, but anyway...
I got a an Error 9 in the registration/activation process.
Unfortunately, once I had correctly activated my upgrade, I found that the serial number changed to 000-00000000. Thankfully a simple solution exists to this on Autodesk - Services & Support.Which meant means that the registration data sent in for the AutoCAD LT 2008 upgrade product contained a company name that was different from the company name my licenced version of AutoCAD LT was registered too.
Thought I'd share that for what it's worth; maybe other members can get a return on the time I invested finding a solution to iron out these little glitches.