The Fix for this problem is easy i had the problem untill i found out the cause
You Need to go to C:\Documents and Settings\All Users\Application Data\Test Drive Unlimited
Inside that folder there should be 2 files CDkey.txt and radial.cdb, Just delete radial.cdb and leave CDkey.txt in there.
Then start the game and you shouldnt have any problems
in what folder do you find the cd keys at i can't find them no wheres