As oskerw said. Also, they are typically found in prebuilt systems, like for instance a Dell, Acer, ASUS etc.
Be careful though, the warranty is a bit different. Usually you need to go through the OEM (the brand of the PC) to get a replacement, or for technical support. An example would be if I had an ASUS PC and the hard drive failed, if it was an OEM drive I'll have to go through ASUS about it, not the drive manufacturer.