St Andreas Wont load?

[Wild]

Hi all, i recently got my son a new computor running an installed windows Vista (which is not very good imo). He installed St Andreas and was in gaming heaven. Then he decided to download extra cars and a file to install them from a St Andreas website. Since then no load?
We did sys restore,deleted all files he installed,tried everything but no go?
Tried the game on another comp.and worked fine.
The place we bought comp from are not very helpful, they got a sale and thats it as far as they concerned. (pc world).
Not familiar with Vista, cant seem to get into registry or management files?
Also Nortons installed which im not a fan of. Other games work fine just St. ****** Andreas.
Can anyone help?