navigator.userAgent
String to look for:
WinXP: NT 5.1
Vista : NT 6.0
Windows 7: NT 6.1
Windows 8: NT 6.2
Tiny JavaScript function to get user's OS:
function getOSVersion() {
navigator.userAgent.indexOf("NT 5.") != -1 return "XP";
/* all Service packs included */
navigator.userAgent.indexOf("NT 6.0") != -1 return "Vista";
navigator.userAgent.indexOf("NT 6.1") != -1 return "7";
navigator.userAgent.indexOf("NT 6.2") != -1 return "8";
return "Other";
}