How can I get windows xp to boot on windows7

I found a hard drive in my shed and it has windows xp on it with a load of old games on it. I still have the disc for the games but need to get into windows xp to play them they will not install on windows7. I have the hard drive in an enclosurer so I see the games and the xp folder now I just have to be able to boot the computer into xp, is it possible? I no longer have the windows xp disc, got rid of it years ago, wish I did it would things so much easier. Oh Well. Thanks for your help.