To install the operative system is a bit annoying but the most important thing is to make a backup of all important data because the new installation will delete everything.
If you don’t know how to do, maybe is better ask to someone that knows better. Basically is needed to boot the computer from the installation CD and follow the steps on the screen. Not really difficult, but in some cases can be a real PITA. For example, with some laptops where specific drivers are needed.
But the most important thing, as I said, is to backup all important data (for example e-mail settings, browser bookmarks, pictures, etc).