Update on my situation: so far, nothing is better.
I did a complete reinstall, but did not install Seasons, because right now, I'd be happy just to get SN and Pets to play again. I used a new game, but I did put my old families back in there. It crashed as soon as I moved out of edit town mode into play mode (my game mostly crashes in play mode). I was too tired and upset to try anything else last night (I've been trying to get my game to run for about 2 days now). I figure my save files are probably hopelessly corrupt now by whatever the problem is, but I am not ready to give up my sims yet!
It did occur to me though, that perhaps something is wrong with one of the sims. I made one family in Seasons before it crashed. I was placing this family into my brand new town when the game crashed. I don't even know if a sim family can be corrupt, but I certainly hope this is the case, because I would be willing to give that one up if it would get the game to run.
And I should stress again I am no longer even trying to run Seasons--I'm just trying to get my old EPs to play.
So tonight, I'm going to see if I can run SN with the town with my sims in it (without the "Seasons" family). If I can eventually get SN to run again, then I may try adding Seasons again, or I may trying uninstalling SN and just running Seasons and Pets and seeing if that works.
But please, if anyone has any suggestions, I would love to hear them. Is it even possible that a family file is corrupt so putting my old Sims in a town will make the game crash?
I'm really frustrated!