Ok...if you only have on Operating System installed on your PC, then you want to use a boot disk to boot into command prompt at startup, or if you have a Windows XP Installation CD, boot into that and start the Windows XP Recovery Console. In both, you'l get a command line interface where you simply type 'FORMAT C:' where C: would be the letter of your Hard Drive.
If you have Windows XP and wish to install if fresh, you can simply boot into the Windows XP Installation CD and select to install Windows XP. You can then select which partition to delete or modify, and Windows XP will automatically format before re-installating.
If you are trying to rid of problems in Windows XP, then you could also do a Windows XP Repair. Simply boot into the Windows XP Installation CD, select Repair and it will only re-instal system files...but not touch personally created data files. However, this method can fail to fix issues in some situations.
|Desktop PC| |Macbook Air 13" (Mid 2013)|
|Windows 8.1 Pro (x64)| |AMD FX 4100 Black Edition 3.6 GHz @ 4.0 GHz OC| |Arctic Cooling Freezer 13 PRO| |Gigabyte Nvidia GeForce GTX 560 Super Overclock 1GB| |ASUS M5A78L-M/USB3 Motherboard| |Samsung 840 Pro 256GB SSD SATA III HDD| |Kingston HyperX 8GB PC3-14900 DDR3-1866| |OCZ ModXStream Pro 700W Modular PSU| |NZXT Vulcan Micro ATX Case|