Well, if your mom's computer is really that lame, it's probably old, which means that it's likely piled sky-high with dust. Grab an air compressor or something and clean it out. It might fix your problem with Sims 2 as well, since heat seemed to be causing my own instance of that problem.
By the way: if she has the money, BUILD HER A BETTER COMPY ALTOGETHER. Sims 2 probably runs poorly anyways due to the lack of ram. 'Cause, seriously, most of us have at least 512MB.
I've checked the heat, and it is not the problem. I certanly doubt dust plays any role here too, as the compy was cleaned a few months ago. And btw, Sims 2 runs perfectly on 256MB.