i would recomend formatting it. you can do this through windows as long as it doesnt try to boot the os on your friends hdd. or if you have the xp cd you can put it in your cd drive, and restart the computer (make sure your bios is set to boot from cd before boot from hdd) and it will launch the windows installation stuff, which you could use to format. no real advantage here except you can reformat before you get to windows.
as for the cable unless you have one lying around my advice would be to buy a new one. try www.newegg.com