On one of my newer machines I haven't seen and I got it 2 years ago. This includes a 1.5 years in XP and .5 with Vista running on it. As for viruses and such honestly if you're a virus maker trying to make money. Who do you target the 1% of users or the 99% of users. It think this is pretty straight forward from a business perspective. Just look at the FF and IE thing. FF has slowly been having more exploits posted as it has grown in market share. Also another thing to say linux or any other os does not bsod or crash you clearly have not been running them long enough because I've had numerous linux boxes crash. Their reasons for crashing? Well one was random kernel panic never could figure out why solution was upgrade th kernel. Another one it lost a hard drive and instantly it locked up. When coming back online it was hardware faulting like crazy till the drive got fully removed. So basically my thoughts here. Windows is the most popular because at the time it was released it was the most user friendly operating system out there. It made a name for it self and now everyone just uses it now even if it may not be the best operating system depending on the person you ask.
Not a whole lot, besides Firefox, iTunes, Windows Live Messenger and stuff. Runs smoothly with all that and Aero enabled. World of Warcraft works okay too.