|Do you have an XP install CD?|
If not there is no point continuing with this process.
As BIOS menus are different for each motherboard we cannot give you exact instructions on how to navigate the bios of your new system. As long as you do not save any changes when looking around you will do no harm.
If you do have an XP Install CD available to attempt a repair install then I suggest you disconnect the Win7 hard drive first so you cannot accidentally trash it.
Next. Connect the XP hard drive and boot up into BIOS. Navigate to the Boot Priority or boot order section of the menu and make sure CD is set as first boot device and IDE harddrive as second. You should note what the original settings were so you can restore them once you have finished messing about with XP.
Insert your XP CD and then save & exit the BIOS. The system should then boot with a "Press any key to boot CD" prompt so you can perform the repair install.
Of course all this presumes that there are XP Compatible drivers available for the various components of your new I7 machine. It is not uncommon for things like sound, graphics, network etc to have no XP drivers available so even if you get XP running, half the peripherals won't work. So please ask yourself the following questions:
Do I have an XP Install CD?
Are there XP Compatible drivers available for the hardware in this new machine?
Am I confident I can restore the machine to correct function of Win7 if messing with XP turns pear-shaped which is likely to happen?
Goin' Fishin' (Some day)