After adding alot of custom content in one session, the groups.cache file can and often does get corrupted. Go to MyDocuments/EAGames/TheSims2 and delete groups.cache. The game will make a new one the next time you run it.
If you downloaded alot of hacks or mods, some of them might be conflicting with each other. You can unknowingly install hacks/mods by downloading custom houses and lots, the creator may not know or realize he/she is including them with download. The Sims2 Exchange is notorious for this. Download and run the Sims 2 Hack Conflict Detection Utility:
http://www.simwardrobe.com You should also download the Sims2Pack Clean Installer from here:
http://sims2pack.modthesims2.com/This will help you sort out your Downloads folder and see what's what. Items highlighted in red are hacks/mods and items in pink are duplicate files.
Alot of custom content relies on the use of CEP. You can get that here:
http://www.modthesims2.com/showthread.php?t=92541 I've seen posts where people have said they have over 14000 files in their Downloads folder and over 10 gigs worth. They've also said it takes upwards of 45 minutes just to load their game. The more custom content you add, the longer it takes to load. Alot depends on how much RAM and disk space you have on your system too. There's no known limit to the amount of custom content you can have, but you have to make sure the downloads don't conflict with each other.
Moving the Downloads folder to your desktop and then slowly adding files back into it is a tried and true (and time consuming) method. Eventually you will find the problem file(s). The above steps should help you find them a little quicker.