Get the XP ISO , Burn it and Install it , Choose the default drive where windows 7 is installed , it will ask for formatting partition go ahead with it and install it would overwrite it.
Things to Note:
1) there might be drivers issues as windows 7 includes drivers in it (Mostly all ).
You might need to download individual driver if available.
Also Windows Xp Official Support will end soon (april 2014) keep in mind that also.
2)I suggest you duel boot to xp or use VM for xp.