Right click "My computer" select "manage"
When the window comes up
Under "Storage" click "Disk Management" folder
Select partition you want to format in partiton window
Right click for menu
Also you can use a standalone boot disk that came with your drive or you download.
You can't formate the partition you use to boot with, or the partition that has the system files on it from the system itself.
You will have to use a 3rd party product, or the Window xp CD.