Welcome to the world of PC's.
Sadly what you are experiencing is what is know as a Product Trial. Meaning that when you bought the PC you were only given a trial of Office for use for a certain amount of time. That time has now expired which is why it is calling for a Product Key.
Also sadly to get this key you must purchase Office. It is not the same product key listed on the laptop as that is the license for you Operating System (Windows Vista).
So your options now are to either purchase Office to get the key or to use a free alternative like OpenOffice
OpenOffice is the free alternative to Office from Microsoft. It can read all .doc, .txt and many more files which you have already created. If you have used the Word default .docx file extension which is used by Office 2007 then you will have to find a .docx to .doc converter. I am uncertain if the current version of OpenOffice can support these file types by default at this time.