Do you just want to format it or re-install Windows? Do you have the XP disks or a restore partition on the laptop?
If you just format the C drive, Windows won't start, even if th emain OS part was loaded on E:. If you have files you want to keep, you can move them to the E: partiton, format C: and install Windows on that, and E: will stay as is. Just don't accidently delete E as well.