Installing MS Office already on PC

I have bought a new PC, with XP installed and a lot of files copied from my old PC. Whenever I try to read a Word file I get asked if I want to install Office and asked what my key is. I do not know the key. I am told one has to buy it from MS; however, my inspection of the MS site shows how to buy via download and what to do with a CD, but not how to get a key for installation when the product already resides on the PC. Can anyone read the MS site better than I can?