Hello LynxGuy,
In this case, it wasn't Microsoft's idea to do it this way. It was the Large Computer Manufacturers. They demanded that Microsoft make it so that when a customer buys a computer that comes with Windows pre-installed, the customer would not need to Activate it.
So Microsoft was forced to develope the OEM SLP/COA SLP key system.
Computers, which are built by large manufactures that come with Windows Pre-Installed, come with two (2) Product Keys:
A) OEM SLP: This key comes pre-installed in Windows, when it comes from the Factory. This key is geared to work with the OEM Bios Flag found only on that Manufacturer's computer hardware. So when Windows was installed using the OEM SLP key (at the factory) Vista looks at the motherboard and sees the OEM Bios Flag and Self-Activates. (that's why you did not need to Activate your computer after you brought it home)
B) COA SLP: This is the Product key that you saw on the sticker on the side (or bottom) of your computer. It is a valid product key and is the Backup to the OEM SLP key. If the interaction between the OEM SLP key and the OEM Bios Flag is interupted for any reason (Installed a new Bios firmware, for example) all the customer needs to do is change the product key to the COA SLP key and Activate by Phone.
I am pretty sure that Microsoft wouldn't have done it this way if it hadn't been for the OEMs. It's much simpler to just use one product key and have the customer activate it when they got the computer home.
At this point it sounds like your OEM SLP key is no longer able to see the OEM Bios Flag. In that scenario, I would tell you to change the produt key to the COA SLP key, but it soulds like you have lost that key. So at this point, I can only refer you to this support doc: KB811224 "How to identify, locate, and replace a product key" http://support.microsoft.com/kb/811224
Thank you,
Darin MS