Windows isn't exactly known as the most secure OS at the moment, which is why Microsoft has gone to great lengths to improve all aspects of security in Windows Vista. Microsoft has coded Vista in such a way that traditional viruses would have a tuff time of compromising multiple machines, thanks in part to the new ASLR (Address Space [Layout] Randomization) feature. Though no system is perfect, Microsoft co-president Jim Allchin is still optimistic about the new security features found in Vista.
"I'll give you an example: It's my favorite feature within Windows Vista, it's called ASLR (Address Space [Layout] Randomization). What it does is, each Windows Vista machine is slightly different than every other Windows Vista machine. So even if there is a remote exploit on one machine, and a worm tries to jump from one machine to another, the probability of that actually succeeding is very small. And I wanted to do this in Windows XP SP2, but we couldn't figure out how to do it. So then a smart guy here came up with a solution, so we put it in Windows Vista."