As the last few people have stated, disable virus barrier X6 (open virus barrier, go into preferences, advanced, select protection off) then delete the app store var file (use spotlight to find and open console, in console search for "/var/folders", open the error message and locate the line "file://localhost/var/folders/ab/...", now open a finder and on the menu bar click go/go to folder and enter /var/folders then locate your "ab" folder or what ever its called on your machine [its different on everyone's] and move it to the trash. Open the app store and try again.
I did all this without a reboot and Mountain Lion has started to download.
Worries the heck out of me if Virus Barrier works with Mountain Lion, but I'll cross that bridge when I get there.