Heres a good thing to ask!!
Ok normally if you never install anything on your computer you would never have to format & reinstall, but most people install software on there machine and of course unistall it because the program they got sucked.
Here are some pointers to remember when you need to do this.
1. Make a backup of all the files you want to keep. Such as documents, spreadsheets, the pictures of the family dog etc,
2. Any software you currently use, such as Nero or Clonecd or something to that nature, back it up. Make sure to go and find the serial numbers that you paid for and back them also.
3. Any applications or games that you made the settings for, when I say this, something like Newsbin pro, has a file that saves all the settings for it. This keeps your newsgroups together.(Not everybody uses newsgroup, so modify the idea to make it fit your computer.
4. If you use Outlook, make sure you save the pst and pab files if you keep your email on you harddrive. Favorites, etc.
There are other steps but these are the ones you think after you hit the button or type in the command format.
After formatting and reinstalling the apps you decide to, Make a image of it using ghost, driveimage or one of the other imaging programs out there, Just in case a month after the format the computer gets sick and throws up blue and wont stop misbehaving, you can be back up in about 30 minutes or.