Lost Product Ket

I had a bad copy of Windows XP professional – I purchased a new one from Microsoft on line. They gave me a series of numbers – product key.
The new Windows is installed and later I received a C.D. I tried to install it again from the C.D. because some of my old programs were gone i.e. Microsoft Word. It did not accept the numbers I received from Microsoft. Is there any way I can find out what the numbers are – trying to get anything from Microsoft is impossible. I downloaded M. Word reader and it is not good. Can anyone help?