Can downloading sims2 cause problems for your PC

Well, of course it can. If your PC is already operating slowly, I'd recommend clearing it out a bit before downloading because it requires a lot of processing power to run the grahics. Otherwise if your PC works fairly well, then you might not see any changes, take into exception it may be a little slower. But it's miniscule and worth it for what you get.
Also, be careful if you are downloading from online unless it is directly from the EAstore, otherwise...
Viruses are a big problem. So, the best way to do it is from the disc. And now they're making tons of 3-packs for only $20 dollars, so it's really cheap.
Hope that helps ^_^