|You do realize that you cannot format C: from C:.|
Second, if you do format the computer you risk loosing the hidden restore partition .
This will leave you with a computer with no disk drive and no way of installing a new OS as you also don't have an xp cd.
The best option is to restore to factory settings option
A Google search "how to restore (your make and model) will save you a lot of headaches.
In simple terms it will format C: from the hidden partition and reinstall a fresh copy of xp.