Resetting the Nero BurnRights has worked for a lot of people with Windows XP, but not for Vista. In the Tools folder of Nero you should find Nero BurnRights. Run that and click the "Reset Burn Rights" button and restart.
If that doesn't work, some people seem to have luck with editing the registry or messing with filters or gear drivers. Check out this discussion starting with Tytus (he came up with the Nero fix) on March 22:
http://discussions.apple.com/thread.jspa?threadID=1939904&tstart=0
If you don't have Nero, check out the about thread too. I think some people have found a way to get just the Burn Rights program.
Hope this helps!