Accidentally in Love is a song by Counting Crows that was written for the Shrek 2. It appears near the beginning of the movie.
Lead singer Adam Duritz (from Billboard): "I was really struggling with it. I generally don't write songs on demand, and I almost got to the point where I thought I wasn't going to do it. They just told me that the song had to be uplifting. They actually said, 'Don't write a song about Shrek. Write a song that's about you.' The funny thing is, the song ended up reflecting a lot of what was going on in my life at the time: falling in love with someone you're not supposed to fall in love with because it's inconvenient. My songs for Counting Crows are mature and generally don't get a chance to reach kids. To be part of something like that is pretty cool." (thanks, Mike - Darkside of the Moon, for above 2)
The girl in the video is Ashley Roberst from the Pussycat Dolls. (thanks, Stefani - Amherst, WI)