You can mute the windows vista start-up sound.

The Microsoft Vista team heard its supportes loud and clear when it came to discussing there plans to include a default start-up sound that plays as you boot-up Windows Vista:

Nick White from the Vista team said " you said you wanted to have the opportunity to mute it if and when you felt it necessary. Your collective voice truly made a difference in this matter, as we've incorporated a way for users to turn off the start-up sound when it's more appropriate to boot silently -- be it when you're mobile, at home or in the office."

Here's a exclusive shot of the control panel that will allow you to enable/disable the start-up sound: