Most of the world uses Windows XP/Vista.
Here's an overview over the Operation System Market Share in April 2008
(from Net Applications)
Windows XP 73.92%
Windows Vista 14.23%
MacIntel 4.35%
Mac OS 2.66%
Windows 2000 2.30%
Linux 0.67%
Windows NT 0.65%
Windows 98 0.54%
Windows ME 0.29%
iPhone 0.14%
Windows CE 0.06%
Pike 0.03%
iPod 0.03%
PLAYSTATION 3 0.02%
PSP 0.02%
Series60 0.02%
Unknown 0.02%
Hiptop 0.02%
Windows 95 0.01%
Nintendo Wii 0.01%
Web TV 0.01%
SunOS 0.01%
WinCE 0.00%
FOMA 0.00%
http://marketshare.hitslink.com/report.aspx?qprid=10