Easier way than using the selection tool...assuming the logo is square which I think it is, View > new guide > 50% horizontal then again for vertical. This will make 2 guides which cross in the centre. Then select the shape tool from toolbar (right side, think its 2 from bottom from memory). If it's not the circle by default click and hold to see it. Place cursor in centre over where the 2 guides meet, hold alt and shift and drag until the circle fits yours. Window > paths...in this box at the bottom theres a circle made of dotted line so to say. Click this and the path will become your selection. Press M on keyboard, right click and select inverse, then delete the selection.
Now that I write it, it looks very long winded but it takes less than 30 seconds and is more accurate and easy than trying to draw a circular marquee.
*edit* oh wait the other method was quick selection tool...well mine would give you a much neater edge than that