Can viruses ever do anything good for your body? are there any beneficial viruses? do they all just attack your cells?

Depends: This depends on what you consider a benefit.We take measles/mumps and rubella viruses, weaken them to the point they can do no harm and use the resultant material as a vaccine, against the natural viruses.We do the same with flu virus, but the injected material is dead.Many of the minor virus infections we have help build antibodies that cross protect against other similar germs.