If you are given a machine by a colleague that they no longer need and wish to re-install it then encountering the "This machine was encrypted with Bitlocker" (not the exact words) screen at the start of the boot is not the greatest experience.

Assuming you don't want any of the content that's already on the hard disk then the easiest way to rebuild it is simply to boot off the network if you have a service like Windows Deployment Service (WDS) or Remote Installation Service (RIS) available to you.