If Windows 8 will be released on 2012, I don't think Windows 7 will kill XP before that. Yes, its just a matter of time but how long this time is uncertain to us all.

I mean, we still see people using Windows 2000 or even Windows 98 today that Windows 7 and Vista are dominating them in terms of technology. XP will die slowly and eventually reach a similar status that Windows 2000 and Windows 98 are currently in, but I believe we will still see people use Windows XP on 2015 onwards.