Having linux won't help protect your XP from viruses at all, unless you consider that as you become more comfortable with linux, you'll use XP less and less, and the less time it is on, the less chance it has to be infected!
If you XP were to become infected, you might be able to use linux to repair, but writing to anything formatted with NTFS is a questionable proceedure. Doing so will usually work, but writing to NTFS can bork up the whole partition and make it worthless.
Usually when people need to resuce their windows install, they use a live CD like Knoppix, or the rescue CD. From what I remember, usually the only way to get a virus removed was to run a detection tool and then remove the virus. Windows usually has to be on for that to happen, although if the virus depends on some executable or .dll file, you could use a rescue disk to remove that file.