The Sims 3 is rather stubborn when it comes to assimilating new information. For whatever reason, it prefers to have a heart attack and go through a revival process than to just accept what is going on and run with it. I've done a lot of testing to see what causes the crashing, and from everything I can tell the problematic files are the ones that over-ride original programming. Adding files like custom meshes and patterns which are in addition to the base programming don't seem to cause any problems. However, files like default replacements and mods can be a real pain in the rear if you don't take the following steps:

(Please note, these instructions are for a Windows XP computer. I have no idea if they are different on Vista.)

1. Install said file to wherever it goes in your packages folder. (If you have no idea where your packages file is, you probably don't have one, so visit this thread at MTS and download Delphy's Install Helper Monkey. Installing this little program will automatically add a folder structure as well as the configuration file you need to have custom content appear in your game. And since no one should NOT know where files are being installed on their computer, you can find this folder structure within your Sims 3 installation folders, which typically is C:\Program Files\Electronic Arts\The Sims 3. The new folder is Mods and within that is a Packages folder which is where your .package files go.
2. Navigate to My Documents\Electronic Arts\The Sims 3 and remove the file named scriptCache.package. This file stores information for start-up and for reasons beyond my comprehension needs to be generated newly each time a file that goes against the original design of the game is installed. (I don't recommend deleting this file at first, just move it safely away, and if all goes well with the following steps it can be deleted.)
3. Start your game and don't click during the movie to disable it, instead let it run all the way through. The game uses the time the movie is running to regenerate the file you have just moved out and update it with the new information.
4. Now just wait to see if the game loads or if it crashes to your desktop on the second ray of loading your town. If it crashes to the desktop, restart the game and more than likely this time it will go ahead and fully load. If it doesn't, restart your computer and try again. Most of the time if the game is going to crash to the desktop it will only do it once, but once in a while it gets on an extremely stubborn tangent.