Remove Viruses?

my computer is old and slow, i recently had it internet enabled and now i have virus's, trojans, spywear and all that fun stuff, what id like to do is format my computer and reinstall windows xp but i dont have the windows xp disks as i recived the computer from a family member, i asked them to locate the disks for me but thay had no luck finding them, the family member that i recived the computer from had ALL KINDS of programs on it and none of them i use or need, so like i said id like to format and reinstall with hopes it will help prformance as well as get rid of the virus's and all those unused programs, so to make a long story short, i just need to talk with someone who may be able to help me, my email is harryfreeman84@hotmail.com so i hope to hear from someone soon, thank you,i guess i should say that im trying to figure out how to format and reinstall windows xp "with out Wxp disks" and mozilla firefox and from there i should be able to handle the rest