Help Computer Infected with 30 Viruses!!!!?

ok so my other computer was infected with 30 viruses with like 6 or 7 trojans Email Worms, and all this other crap and this all happened when my sister was using the computer now Windows security found this out so i called my dad and he said ( Yesterday 4/16/11) to remove the viruses and i tried however we are using the unregistered version of Windows sec (oh and we are running Windows XP Proffessional) so it needed a registration key which i could not find so i shut the computer off) now my dad is home (4/17/11) and he tried but we have to buy the dam software but we could have a key logger and Screen looker thing that will steal the damn information oh and mind you this is all on my Sisters Profile if i log into mine or others we dont get a virus warning and all the viruses are in her documents) so now im sure that it WILL spread to the entire computer and I really need advice on this because i need to use that computer for a project. :( so we cant use Windows Security however we have CA Anti Virus and MACFEE( what that is xD) but CA hasnt picked anything up yet when Windows security has. HALP!!!!