TurnipBlaster :
can you please explain? Can't i just transfer my old OS for regular use?
That old HDD with XP actually booting in your new hardware is very, very doubtful.
Does your new equipment have drivers for XP? Probably not.
Moving a drive with an existing OS is very much of a 'sometimes' thing. The father apart the hardware is, the less likely it is to work.
I have two Compaq laptops on my desk, almost identical. Moving the HDD with Win 7 from one to the other...no boot.
You can download a valid Windows 7 ISO, burn to DVD, install, and use it for 30 days.
Maybe that will give you enough time to save up the $95 for a new Windows 7 DVD and license key.
Download here: http://www.w7forums.com/threads/official-windows-7-sp1-iso-image-downloads.12325/