I've found Acronis TrueImage (Home edition) to be as unobtrusive as it gets on Windows. It starts and stops when told, and so far plays nice with other disk IO.

I've found Acronis TrueImage (Home edition) to be as unobtrusive as it gets on Windows. It starts and stops when told, and so far plays nice with other disk IO. That's not a formal review -- I just haven't noticed it running the way I've noticed backup utils I've used before.