OneStat.com, today
reported that Microsoft's Windows operating system has a global usage share of 97.46 percent. Microsoft's Windows still dominates the global operating system market. The global usage share of for Apple's Macintosh operating system is 1.43 percent and is the second most popular operating system in the world. The three most popular operating systems according to OneStat are: 1. Windows 97.46%, 2. Macintosh 1.43%, 3. Linux 0.26%. All numbers are an average of the last 2 months. The other big statistics company, StatMarket, placed (in June 2002) Microsoft at around 95%, MacOS at 2.4% and Linux around 0.35%.
If you want to see better stats go here
http://www.google.com/press/zeitgeist.html
and see what operating systems are used to access google.
Again, this can't be definite number because many people change their refferers to access some sites. IIRC, Opera on Linux is using IE as a refferer by default