And tell them explicitly that if their computer tells them they have a virus, they must not try to do anything about it and call you immediately. 99% of the time it's a fake pop-up and they don't want to look stupid so they follow its "friendly instructions" to "get rid of it" and end up making a mess.
Other than that, take away their admin privileges, set up 2AM auto-updates (or manual, if you're there often) and tell them to only store personal files in one specific directory, which is synced to something with CoW or daily backups (and then also sync the desktop just in case).
As for e-mail, I set up my grandparents with one e-mail for people they know and a gmail for everything else (like website registrations). That way, the personal address never* gets any spam.