You can't format C drive while you are in windows.
It must be done using your OS disk. You will need to change the boot sequence in bios.
To change Boot Sequence
in your BIOS
Reboot the system and at the first post screen (where it is counting up memory)
start tapping the DEL
This will enter you into the Bios\Cmos
Find the Advanced
area and click Enter
Look for Boot Sequence
or Boot Options
and highlight that click Enter
Now highlight the first drive and follow the directions on the bottom of the screen on how to modify it and change it to CDrom
Change the second drive to the C or Main Drive
Once that is done then click F10 to Save and Exit
You will prompted to enter Y
to verify Save and Exit
. Click Y
and the system will now reboot with the new settings.
for step by step instructions on how to partition, format, and install XP fresh