Windows 7 has the best security of any system (including Macs, which I still occasionally have to use) I've ever dealt with. Despite all the marketing, they don't really need additional software for security. I still run things like Spybot but they always come up squeaky clean.
The days of Macs being malware and virus free are a thing of the past. Virus writers never bothered with Macs because there just weren't enough to bother with but now they have discovered them. My employee gets malware on his Mac (admittedly not much) but I haven't had a piece make it through since I've gone to Win 7.
