I gave you a +1 for this, but I'm wondering...
What if Microsoft built an OS on top of UNIX alongside their flagship OS, and did a gradual migration along the way?
As I understand it (I wasn't a Mac user until about 3.5 years ago), OS 9 was full of viruses, so it wasn't that they had a smaller marketshare, it was that the OS was too vulnerable to attacks.
Enter OS X, powered by UNIX. Now you have a much higher grade security model, stricter permissions, and only *necessary* times to enter your password (unlike Windows where it's always asking for your permission to run this or that...even in Windows 7.)
So 10 years later, how many viruses does OS X have? How much spyware/malware? And sad but true, while viruses were common in Windows XP, these people are writing even MORE malware than ever - we're removing viruses from customers' computers left and right at the store I work at. Windows XP, Vista, 7, doesn't matter.
I honestly think UNIX is what helped curb the virus issue on MacOS, and would *significantly* reduce the malware that Windows machines are subject to every day. Take UNIX/Linux itself - I know of VERY few viruses (really they were more like trojan horses that required root access) that were affecting these systems (serious server/mainframe-class machines - SunOS/Solaris, AIX, Linux, *BSD, etc.)
That's my opinion, and I think Microsoft would be wise to consider doing something like this...not just for the short term, but I think as Apple did, for the long haul.