Xbox -> Hacking -> Alternate Bios:

The stock bios in the Xbox has many security restrictions built in, such as not allowing unsigned code, backup media, or a different hard drive. Loading a hacked bios file allows users to override these security features, as well as customize the flubber animation and Xbox logo seen when the Xbox first boots up.

The most popular alternate bios files are:
EvoxBios
XecuterBios

These bios files come with many features by default, but still usually require some customization to work properly on your Xbox. The boot colors, dashboard location, fan speed, and drive partition sizes can be easily modified using one of the bios tools.