Hard Drive Switch

I'm building a new computer and to save money I'm using my old hard drive. I need to know the steps to put this on my new computer..do i need to completely wipe the Hard Drive and install a new OS? or can I just switch it over  and have it run on the new system? It is the oem version of Windows 7, but if I have to re-install it will have to be XP since thats all I have...Please help..everything in the computer is new...I just want to use my old Hard Drive to save money, I don't have any important files on the Hard Drive so if wiping it out is easier let me know. (I would prefer to save Windows 7 for Black Ops 2)

you can get an iso from an official source, and use the same key, I think... you would be better off with linux mint rather than xp.

Should I still wipe the Hard Drive first?? Just to make sure there isnt any mixing of 7 and Xp ...or linux if i take that route.