I use two operating systems: Windows XP and Ubuntu Linux. I still use Windows XP to develop for Microsoft's .Net platform. If I had my choice, I'd never touch Windows with a ten-foot pole.
Here's a short comparison of the unpatched security holes in the operating systems. (These security reports are updated daily).
Code Red: Windows XP ("extremely critical" rating as of 7/9/2009!):
Code Yellow-Green: Windows Vista ("less critical" rating as of 7/9/2009!):
Ubuntu Linux 9.04 (no unpatched security holes as of 7/9/2009):
This snapshot is a fairly common view of the three systems. Also, it's worth pointing out, these security holes didn't just pop up. They've been sitting out there the whole time (hopefully unknown) since the release of the software.
Well, now Windows XP is asking me to restart my computer. I'm NOT joking. :)