Limits in limited accounts
Theoretically, some sort of restricted account should help. Although, I wouldn't consider it to be a guaranteed fix...just something free, easy to do, that might help a bit.
Unfortunately, though, these fixes only go so far. I, myself, would not trust an XP machine connected to the Internet. Nor, for that matter, would I trust Vista. I've read of at least one account of a person who's had problems with a Vista system picking up malware.
It seems to me that there is also another problem with limited accounts I'd half forgotten about: some software won't run properly on XP Home's limited account. So, it may be that Windows XP Home users, which (I'm guessing) would be most home computer users, would be better off sticking with regular accounts. (I should have remembered this earlier--but frankly I use Windows so little that it's not easy remembering some of these details sometimes.)