Have *all* of Dell CDs as they will contain your XP OS and drivers plus the bundled s/w if so brought. As for drivers, get all that before reformat, if no disks. If you don't have all the disks, then contact Dell for replacement. They maybe free or small cost to do so. Once you got all that done, then reformat your main HD. I like to use the pgm. KILLDISK from the same website name. This will clean entirely your whole HD, so be sure you save any "data"(just data) you care to before formatting. I mention killdisk to totally wipe out everything, then load the XP disk and start the reload process. Dell, has the instructions to reformat or relaod your OS, find your model# and get the details. The killdisk pgm. is an extra step, but you won't have the mess you've gotten as you explained. And YES, you may need to contact MS to re-activate your OS if need be after all is said and done.
As for back-ups, you may have also backed up the virus too, so if the problems come back, you basically reloaded them too. Thus, only data should be saved, not whole sections of pgms. etc.. It's better to reload a pgm. fresh then place the saved data back into the directory, rather than a while directory with pgms.. If this doesn't make sense it will should you proceed anyways.