Although not a totally unbiased survey, this survey does present some interesting numbers. With over 800,000 machines being polled (yes, machines... not people), you can see that most of them have a single processor, half and half are running Intel and Amd and other information like this. What's interesting is to see the Windows XP breakdown.
Windows XP SP2 -- 73.03%
Windows XP SP1 -- 12.86%
Windows XP -- 6.90%
Now when I say that this survey is somewhat biased, I mean that most of the people running this should have a machine that is capable of running Half-Life 2... which is a beast of an application. For the most part, you'd kind of expect people with such a machine to be on top of their updates, but clearly they aren't. You can also see that it's obvious that over a quarter of the people are not running the “latest” operating system and of those running XP, don't have the latest security patches. That's somewhat scary if you ask me. Anyway, take a look at the survey results yourself...