This link should help you out :

http://support.microsoft.com/default...b;en-us;320397

and:

http://support.microsoft.com/default...b;en-us;816793

If you do not have a boot disk or the WIN XP cd, you will most likely have to reformat the drive... But the fact you cannot find the OS disk could be a problem when it comes to reinstalling...

good luck!!!