If I build a computer, can I put my old hard drive in instead of buying a new OS? So basically, instead of buying windows 7 for my new pc, could I just put my old hard drive or install an image of my old hard drive onto the new one and boot up?

I'd imagine there would tons of driver issues and stuff, but as long as it booted up, I'd think it'd be pretty easy to reconfigure to the new hardware pretty easily.