Ed Bott posted an article on the OEM version of Windows 7

I’m talking about OEM System Builder licenses for Windows desktop editions. If you look at any online shopping site that caters to PC enthusiasts, you’ll find these copies displayed alongside the upgrade and full license packages that Microsoft says retail customers are supposed to buy. My friend and fellow Windows expert Paul Thurrott just posted a thorough look at the Windows 7 OEM System Builder package, complete with pictures. If you’re building your own PC or looking for installation media that won’t make you jump through hoops to install it, this product is extremely attractive, because it’s significantly less expensive than a full retail license. The installation media works almost exactly like a full retail copy of Windows, except that it can’t be used to perform upgrades, only a custom (clean) install. After installation, an OEM copy is essentially indistinguishable from a retail copy.

Is it OK to use OEM Windows on your own PC? Don't ask Microsoft