Windows Vista

Robert Fripp created the startup sound for Windows Vista. Per Microsoft, the sound:

• is made of dual ascending 'glassy' melodies played on top of a gentle fading Fripp 'AERO' Soundscape
• has two parallel melodies played in an intentional "Win-dows Vis-ta" rhythm
• consists of 4 chords, one for each color in the Windows flag
• is ~4 seconds long, end-to-end
• is a collaboration between contributors Robert Fripp (primary melody + Soundscape), Tucker Martine (rhythm) and Steve Ball (harmony and final orchestration)

Also:
It was our goal for the sounds to be just noticeable enough that if they weren’t there you would miss them, but they’re subtle enough that they don’t get in your way. Just like the visual cues of the new Start button, the audio cues of the new sounds are "rounded" and "translucent." The default sound scheme in Windows Vista is intentionally much gentler and softer than in previous versions.

You can listen to it here.

BTW, Brian Eno created the startup sound for Windows 95. Eno says:

Q: How did you come to compose "The Microsoft Sound"?

A: The idea came up at the time when I was completely bereft of ideas. I'd been working on my own music for a while and was quite lost, actually. And I really appreciated someone coming along and saying, "Here's a specific problem -- solve it."

The thing from the agency said, "We want a piece of music that is inspiring, universal, blah-blah, da-da-da, optimistic, futuristic, sentimental, emotional," this whole list of adjectives, and then at the bottom it said "and it must be 3 1/4 seconds long."

I thought this was so funny and an amazing thought to actually try to make a little piece of music. It's like making a tiny little jewel.
In fact, I made 84 pieces. I got completely into this world of tiny, tiny little pieces of music. I was so sensitive to microseconds at the end of this that it really broke a logjam in my own work. Then when I'd finished that and I went back to working with pieces that were like three minutes long, it seemed like oceans of time.