The HardwareSecrets guys are out to take the mystery out of buying an OEM PC. I personally am likely to not ever buy one again, but you may have family or friends that you can't convince to just let you build them a PC, so you might want to give this article a look.

"If you are thinking on buying a new PC it is good to follow our tips in order to be not deceived. In this tutorial we will talk about the most common problems you can face when buying a new PC and also we will teach you how to check, without opening the computer case, if the PC you bought is really the one you ordered."