Meggsie04,
Assuming the computer is still operational, use your favorite search engine to look for "Magical Jellybean Key Finder." Download and run it, and record your installation Product Key.
At one time, the End User media replacement form had a section for replacing a lost CoA, but the current version of it does not seem to have that section anymore: http://support.microsoft.com/default.aspx?scid=kb;en-us;326246
For great advice on all topics XP, visit http://www.annoyances.org/exec/forum/winxp