OK - start by reading through here:
http://www.mcmcse.com/windows_xp/guides/additionalinstall.shtmlHow
ever if you just want to do this because you don't have access to a bootable drive or USB stick you might want to consider removing the hard drive, formatting it as FAT32 and bootable and copying the installation files from an XP Install CD (the contents of Folder i386). You can then put it back into the computer, boot it and at the DOS prompt run SETUP.EXE
