WinGUI: Update the installer to abort if running on windows xp or earlier with an error message. I don't have an XP machine / VM to test this on anymore, so if it doesn't work, let me know.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6596 b64f7644-9d1e-0410-96f1-a4d463321fa5