How can I make a Windows PC bullet-proof for home users?

Problem

I realize that virus-proof on a Windows PC is far fetched, but in the interest of keeping time spent as the "family-tech-support" to a minimum, I am looking for ideas to lock a computer down to the point that it is very hard to collect ad-ware/spyware, malware, or viruses. Assume the user is my mom, who rarely, if ever, needs administrator access and mostly uses the computer for MS Office and web browsing.