The User Agent from the browser is not something I would rely on for anything, We all use it for statistics, but we know they're not 100% accurate.
I use firefox and regularly spoof IE for some sites that don't like it, my regular UA is:
Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.3)
Gecko/2008101315 Ubuntu/8.10 (intrepid) Firefox/3.0.3
I sometimes use a firefox extension to change it to:
Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; MS-RTC LM 8;
.NET CLR 2.0.50727; .NET CLR 1.1.4322)
when you are looking at it, you would need to parse the different parts, the OS is the third part of the semicolon-delimited values between brackets.
