HD Swap Os no go

I take the hd out of the windows 7 machine and place in the hd with windows xp and it boots to where it says windows xp and restarts. The only thing im trying to do is take my xp hd and put it in a win7 machine. From wht i have found the pc will run xp smoothly but i am having toruble getting it to boot. i figured it would just assign generic drivers but it didnt. so is there anyother way via repair install or something i copuld try before a reformat? :confused