Here is a snapshot from Sitemeter of visitors to The force that through... in terms of operating systems used. Two things standout. First MacOS is running about 7%, as is Vista. Windows XP is still the most popular system, most Windows users choosing not to upgrade on their current machines. That is quite understandable as older machines can't handle Vista's graphics requirements.
For the record, I run Mac OS at work, Vista Home premium on my home desktop and XP on my laptap. The laptop most certainly would choke on Vista. Since most of my visitors come via Google-roughly 75%, looking for answers to such questions as "Why are plants green?" or "What is weasel sex?" or.. How do I do problem 5 on my evil genetics test?", these may represent a pretty good cross section of users.