DISCLAIMER: I am no expert, I just recently had similar problems and did some research online.
First off, remove your downloads if you have any. You can pull the whole folder out to the desktop so you don't have to delete your downloads.
Go to "My Documents" -> EA Games -> The Sims 2 you should see a downloads folder. Move it to the desktop. Try to start the game, if it loads it's a bad download. If you want your downloads you'll have to carefully add them back one at a time to find the problem one - then delete that one.
There are some hack conflict detection utilities available online, however I think they miss some conflicting hacks and you have to do some manual detecting.
If that doesn't work, go to The Sims 2 folder as above and delete the file "Groups.cache". The game will recreate this file and if there's a corruption in that file it may be what's keeping things from loading correctly. Deleting a game file sounds scary but I did it and my game works fine now. I found this recommendation online but I was searching so many places I can't remember where I found it, possibly here or MTS2.
Mine was loading through most of the intro and then closing back to the desktop. This was after a had reinstalled after a bad download corrupted the game. Apparently, it was still corrupted and deleting the Groups.cache fixed the problem. I was all set to start pricing new video cards, too.
Good luck!
ETA: Here's a link to another thread here that talks about deleting the groups.cache file. That must have been where I saw the info.
http://www.insimenator.org/showthread.php?t=38128