As i have built serveral oem versions of windows and i am an microsoft authorized oem distributor i figure i would shed some light on the subject.
Its not a damn tatoo. computers dont get tats they dont have skin
Seriously though, it is a custom bios image that that is stored in an eprom and then when the computer starts up if the bios information is changed then dmi data is updated. then after that the new bios checksum is stored in the cmos.
The oem manufacturer puts this on the motherboard because of all the packaged software in those oem windows builds. There is security implemented on those wondows recovery discs That assures that the software only runs on authorized hardware.
Dell, hp, sony ect. nor the software industy want people running around with a disk than can make dell clones !!!!!!!
This image can however be flashed in windows sometimes. Depends though on the type of bios and if it really has a full fledged bios. some newer oem pcs dont have a real bios. They write part of the configuration to a partition in the hard drive now. As well as the windows recovery, all on a partition.
now ya know.
One time i got an oem computer for christmas and the motherboard wouldnt let me adjust voltages or bus speeds or nothing so i went and got a fat asus board with the same chipset. swapped it in. the real probem was that i had to install the old motherboard again to get the recovery discs to work again then put the new hardware in.
So for guys stuck with oem pcs that want to tweak and overclock but cant due to a lame oem motherboard. get another motherboard with the same chipset and instal it. it will boot right up and and run fine. youll only have to instal drivers for new devices and options that your oem board didnt have...