A reformat will
definately get rid of a worm or any viruslike nasty that's sitting on your system. Unfortunately it will also get rid of everything else on your system like Windows, downloaded files and any programs you've installed. It's definately guaranteed to get rid of the problem but I'd consider it a last resort.
First try to identify the worm and then look for a cleaner designed to clean that particular infection. You can usually find cleaners for just about all worms, viruses and trojans at the major AV sites like Mc Affee or Symantec for free. Use that to clean your system according to the instrucitons on the site you download it from and then install a good AV program to keep from getting infected again.
If all else fails however and the cleaner can't get all the infection or it keeps coming back then you might consider a reformat. If you have a package PC then you should have reinstall disks that came with the PC. Backup anything you've installed after purchasing the PC including downloaded files and software activation keys. Also back up everything you can't afford to loose before using the reinstall disks. Just pop those in and follow the instructions and you'r computer will be restored to the state it was in when you purchased it.
If your computer is a home built like mine then it's a little more complicated since you'll have to reinstall all software by hand but you'll probably know how to do to all that. :P
