Use CopyWipe for Windows. Burn a cd with it then restart the pc and boot to the cd. How to do that depends on your model. Example you press F12 as the pc starts up. See the screenshots of what you will see when it does boot
http://www.terabyteunlimited.com/copywipe.php
At work this is what we use to erase everything on a pc prior giving it away. If you want to instal XP on it again and if you do not have a cd then go to Ebay. There are many for sale.
http://www.ebay.com/bhp/windows-xp-home-edition
If you buy a new pc you can also put Windows 7 on it.
Amazon and Best Buy are selling it online.
If you can start up the pc to Windows then install the free MiniTool Partition Wizard 8.1.1 at http://www.partitionwizard.com/
You can resize or delete partitions without removing Windows.