Returns The string containing the current OS version.
"6.3" - Windows 8.1
"6.2" - Windows 8 / Windows Server 2012
"6.1" - Windows 7 / Windows Server 2008 R2
"6.0" - Windows Vista / Windows Server 2008
"5.2" - Windows XP 64-bit Edition / Windows Server 2003 / Windows Server 2003 R2
"5.1" - Windows XP