Re-install WinXP Home

I had to do a fresh install on an Acer Aspire. I used an OEM CD and the customer’s cd key on the side of the box. When I tried to activate, I got an invalid CD key message. I called the activate by phone number and was told it should work with the cd key. They couldn’t help any further. The computer was purchased from The Source with XP preinstalled. Anyone have any suggestions? No modifications were made to the computer. Everything is original.