I would like to inform you that Nirsoft has got few useful tools. One of them that I use for the software audit is ProduKey. It will allow you to gather the product keys used on computers across your whole domain. And just like the password utility, it is free!
I checked it out, and it is a great way to grab serial numbers of Microsoft products.
You can use the gui interface, and just see the product keys on your own computer:
You can download the utility from here
It also has an unbelievable number of options for pulling information from other computers or even drives where windows is installed: