I wouldn't drop the gamedata folder in, as that is where 90% of the game settings/addons live, its where 90% of the problems are going to come from... in other words, no point doing a clean install just to overwrite it with old files. I'd also recommend making a brand new plr file, etc, because if there is anything in there that expects some file/car/track to be present which is no longer there, then everything comes crunching to a halt. Might be a pain to edit it all over again, but probably worth it.
If I had to put money on it, I'd say there is a nasty track or car that its trying to load/set-as-default at start up (the last one used, or something like that) that doesn't want to load. A barebones install is your best bet for this type of problem I think.
I've seen a similar thing when I copied an old plr file into a new install, and I think it was trying to load that plr, finding that whatever default car/track wasn't on the current install and freaking out. It loaded that profile okay within the game (I think), but on restarting it would go crazy.
Finally, set the game to run with error logging... I can't remember what the command line argument is, but I'm sure someone will. That will hopefully tell you what file it was trying to load when it bombed.
Good luck mate, hoping to see you on track tomorrow (today your time).