App Store won't let me log in

I'm using Lion and I can't update to Mountain Lion because the app store won't let me log in.

I've tried a bunch of things like changing permission settings for certificates marked with a blue '+' to 'always trust' or to 'use system defaults', but it keeps telling me 'connection failed'.

I'm using an admin account so parental controls aren't an issue.

I can access App Store on my iPhone just fine and I can log into App Store using my account on other computers.

(I'm not sure if it's directly related but I also cannot update things like iTunes now either)

I can't figure out whats wrong!

Can anyone tell me what's gone wrong??