Windows/Office Key Checker

Is there any tool or method which I can use to check if a certain Windows Product or Microsoft Office key is valid?

No - the best you can do is telephone the Activation Center and ask them, if it's not already installed.
If it's Windows, and installed, use the MGADiag tool and post the output.
If it's Office 2002-2007, and installed, run MGADiag and post the output.
If it's Office 2010 post a query with your Product ID in the Answers Office forums

I think that there are key finder programs that can do that if you have lost the original Product Keys that came with the Windows and Office Software. Tuneup Utilities also displays these keys in their System Information>Windows Page. Microsoft keeps these keys encrypted in the Registry so they are not easy to find out by searching for them.

But the best is to phone Microsoft and get the Product Keys as NoelIDP has already suggested.