format c: /U (dos command from c: prompt)

 1

Insert your Windows XP installation disc into your CD drive (Home or Pro--it does not matter).

 2

Now as you computer boots a little more it will say "Press any key to boot from CD.." press a key to do so.

 3

The CD will load up a blue screen and then spend a while loading files it needs. When it is finished it will list a few options, mainly "Press ENTER to set up Windows XP." Press Enter or Return.

 4

Now you will be at a screen to select where to install Windows to. This is where you can delete old partitions and format drives. The box in the bottom half of the screen shows all your drives and the partitions that exist on them. Use the Up and Down arrow keys to highlight your "C:" partition and press the 'D' key (if all that shows up is "Unpartitioned space" and you have no C: or D: partitions, skip this step). On the next screen press the 'L' key to finalize deleting the partition.

 5

Now you are back on the screen to choose where to install Windows. The box on the lower half of the screen should no longer show a partition but simply have an entry "Unpartitioned space xxxxxMB." Select this with the arrow keys and press the 'C' key to create a partition on the drive. The next screen tells you the minimum and maximum sizes the partition can be and lets you pick the size. The default size is the maximum, but double check that the number entered is the maximum and hit enter.

 6

Now you will again be back at the choose where to install Windows screen. But this time you will have a partition that looks something like this "C: Partition1 [New (Raw)]xxxxxxMB." Highlight this entry and press enter.

 7

The next screen lets you choose which file system to format the drive with. Choose NTFS as it is faster and more secure. If the drive is brand new and has never been used before then use one of the options that ends in "(Quick)." Or, choose one of the lower down options. Use the arrow keys to select the proper one and press Enter or Return.

Read more:How to Format a Hard Drive With Windows XP | eHow