First off, what brand of computer is it? Name brand (HP, Dell, IBM) or one of te clones out there?
If it is a major manufacturer you should be able to order a replacement OS disk for a fraction of the $$$$$ of purchasing a retail box of either XP or Vista. Contact the manufacturer or check their web sites.
If it is a clone check with the shop you purchased it from and see if you can get a replacement disk from them.
Most brand name and a lot of the larger clone manufacturers have OEM licensing for their software. They have their own version of Windows that will only work with the bulk lot of licenses (Serial #'s) issued to them.
For instance Dell's Windows disk will only work with the software SN included with a Dell machine etc.