Have you tried to clean the virus first? Reformatting
harddrive is a hardwork and time-consuming. You will loose
all the files if you haven't back-up first. You'll also
have to reinstall all the programs and drivers. Try
visiting the Symantec or McAfee website to get the removal
tool for your virus.
Jeremy
I would first follow CaoCao's suggestion and try cleaning the virus first.
But if you've tried this and still have an unhappy machine, check out the
below link for a good explanation on how to clean install XP.