The X-Box is basically a standard PC. However, Microsoft sells it at far below the price of the actual hardware. In order to recoup that cost, software vendors have to pay Microsoft a licencing fee. To protect that fee, they tried to secure the boot processs of the X-Box, so that it would only boot their X-Box operating system. However, a bunch of clever hackers figured out a number of ways around it, which is well documented in The Hidden Boot Code of the X-Box. This article is well written and is very interesting if you have an interest in these sorts of low-level details. For a description of what they should have done, see Authenticated Operation of Open Computing Devices (alternate copies).