For Christmas we bought our mom an iMac. There were a few reasons, but the main one had to do with spyware. I have heard horror stories from friends about going to visit their parents and finding their computers thick with spyware, porn popups littering the desktop, and their parents assuming it was normal. (Matt documented a few related troubles with his dad's computer)
I bring this all up to link this BBC article that cites a study by Earthlink that it had "uncovered an average of 28 spyware programs on each PC scanned during the first three months of the year."
Do your parents a favor and teach them how Ad Aware works... or better yet, get them a Mac.