Eady,
On the COA, if the wording directly under "Windows XP Home Edition" is "OEM Product" or "OEM Software" then the Microsoft link that Carey posted is your best choice because you originally had a genuine MS CD.
However, if the wording is the name of the company that manufactured or assembled the computer, the best place to inquire about a replacement CD is the company itself. For example, I know that eMachines has replacement CDs for many of its models for about $25; Dell has a replacement OS Reinstallation CD for about $12-15, HP/Compaq, Sony, and others sell theirs, too, but availability and prices vary widely.
If your manufacturer does not have your CD, try www.restoredisks.com as one alternative.