I can count the number of system crashes I have had in that time on one hand, and most of them occured immediately after an upgrade while installing stuff.
Because I'm incredibly amused by how well it's been working, I feel I should tell you that I've had one annoying issue with Windows 7 regarding a specific update a few months ago, and exactly one crash since I installed it about two years ago, which was my fault (a looped command script I made that, well, accidentally opened a million instances of Photoshop or something while I was away from the computer).
Nor have I had any virus problems...
*insert generic popularity-versus-infection-likelihood argument here*
Honestly though, after the bug-ridden garbage that was Vista, and fully understanding that 7 isn't any more than a debugged and, in some ways, de-fanged version of it, I've been really impressed lately. I wouldn't trust it in any sort of environment where 99%+ uptime is a necessity (Windows Server 2008 scares me), but that's feeling almost like a statement on "old habits die hard" rather than a comment on their current stability. *shrugs*
I do love penguins though...