Possible To Erase Things Outside Of Marquee Tool?

[user name]

If I make a box or circluar outline on an image and then use the erasure tool, I can only erase anything within that "box", but is it possible to erase anything outside the selected area? What I am tring to do is erase edges of an image using the circular marquee tool, to make them perfectly round. But the only thing that I can erase when selecting the area is what is inside the area, but not the outside that I actually want to erase. I hope this makes sense to someone.....haha

[golemeye]

Just invert the selection

[user name]

Thanks so much. You're my new best friend