Cannot connect to App Store

Hello All

This is my first post so for a quick intro; I'm the Dad of a family of 5 living in the Northwest of the UK, with an iMac, iPad and several iPhones and iPods. I'm reasonable operating Macs after 5 years and would be really grateful for your help as after several weeks of trying to sort the following problem out I remain stuck solid.

When starting App Store on the iMac (only) MAS says "Cannot connect to the App Store". It has a sign-in option, however nothing happens when you press it. When you try updating software MAS advises "An internet connection is required"

- It's only on the iMac; the iPad MAS is fine
- Firewall is OFF
- There is no anti virus software installed that I am aware of. I certainly haven't installed any.
- Use Passive FTP mode is on
- There are no proxies active
- It is however set to bypassing proxy settings for *.local, 169.xxx/xx (numbers shown as x for security)

I have recently installed Mountain Lion 10.8 and at present because of the App Store issue cannot update to 10.8.1

Thanks!

If you log into the guest user, or create a new user and log into that, do you get the same issue?

Problem solved! Unbeknownst to me I was in the Guest Account hence MAS wouldn't update. Someone, sometime must have logged the Admin user out.

Massive thank you!