There's already Music Maker software, I believe. It's for the Xbox, though. But I'm sure it could be ported to the PC since the architecture's so similar. It's not close to what GarageBand does, though. Considering how poor Windows Movie Maker is compared to FCE and even iMovie, I wouldn't much worry about it.

At least not from Microsoft. Sonic Foundry's (Well.. now Sony's

) Acid is somewhat similar to GB, but more to Soundtrack..