Mikhalkov

Pertaining to the battery-
Since this is a hand-me-down laptop, the battery has probably reached the end of it's life span. Batteries typically last around 2-3 years, so if your friend bought it a while ago, that's probably what's going on.

As for the reinstall-
You will need a Windows CD, so ask your friend about it or call up Dell like Ryan said. If you aren't able to get one, you'll end up having to buy one. Anyway, once you have a disk, put it in the CD drive and shut down the computer. Turn it back on and before you see the Windows logo with the loading bar, you will need to hit some key (usually Esc, Del, or one of the F keys) to try and get into the BIOS (Basic Input/Output System). From there, navigate through the menus until you can find the boot priorities of your devices. These should include things like your hard drive, your CD drive, etc. Move the CD drive to be first on the list and save and exit. Then it should boot to the installation CD where you can format the hard drive and reinstall. You may be able to skip this and go into another menu that let's you just select the CD drive and boots.

I know the instructions from off the top of my head are very poor, and this process is hardly intuitive for basic users. Remember, google is your friend, so it would be good if you just googled "how to boot to a cd" or "how to format (XP, Vista, whatever you're using)"