My guess is that your Sophos virus software is too sensitive to things and might need to be disabled until you are finished installing Moodle.
I gave a presentation at last year's GlobalMoot about how I install a local sandbox Moodle on my Mac. I choose to do a MAMP+Moodle install because at that time, I was having problems with the Moodle4Mac product.
Here is a link to some resources that I made available, including a video.