From Halopedia, the Halo wiki
The Windows logo is an Easter egg in Halo 3. On the multiplayer level The Pit, go into the sword room and locate a phone hanging on the wall, bright yellow with a red error message on it stating that it is out of service. Melee it and it will break, over on the back of it, there is a small Windows logo. There is also one of these on Blackout, near the basketball hoop.