Linux Ubuntu - free, nice, fast, secure - that's a choice I'd recommend to anyone.
Agreed. No operating system is invincible but from my experience its very very easy to get viruses and malware on windows even with an anti-virus but I've used ubuntu for a couple of years without an anti virus and had no problems at all and I spend a lot of time downloading warez from less than legit blogs and torrent sites etc which will kill your windows comp pretty fast with some sort of malware infection eventually.