If it is a worm usually it should be infected to make sure Use combofix from bleepingcomputer.com, then go download malwarebytes from softpedia.com or cnet.com and use it, then clean the rest of the virus with superantispyware from softpedia.com or superantispyware.com, and for future protection...
Put the xp cd in cd. Reboot the system set your BIOS to bootup using cd/dvd drive first and it should boot off your cd and after that just install it, be sure to delete the old OS which is win7