Here's what I have from about 6 months of Google Analytics. This is a line-of-business web application. GA is only on the login page.

Visits 5,645

% of Total: 4.25% (132,872)

XP: 62.37%

7: 35.57%

Vista: 1.52%

Server 2003: 0.53%

That Windows 7 number scares me a bit.