What does end for support for a Windows mean?
As far as I understand it means that updates (even critical) won't be available after that date.
But how about a Service Pack. E.g. Windows XP SP2 deadline is set for 11 July 2010. Will Windows XP owners who have Windows XP SP2 version on a disc be able to successfully activate their copies of Windows after that date ? Thanks.
End of support for SP2 is, indeed, 2010. If you want support past then upgrade to SP3. You may get really critical updates after end of support, but that's all. (There is still the occasional critical update for 2000.)
You can stay with XP as long as you like (or as long as it supports all your hardware). It's just that you won't get any new features, including device drivers for new hardware, and - after a while - you won't get any security holes fixed.