Reinstalling the App Store may help. Download and install this combo > OS X Lion Update 10.7.3 (Client Combo)
Then restart your Mac.
How to re download apps from the Mac App Store:
Open the App Store. From the menu bar click Store > Sign In
Click Purchases from the top of the App Store window.
Select which apps you want to re download. Then right or control click where you see Installed then click Install.
Make sure and use the same Apple ID used for the original purchase.
If that didn't help, if you have anti virus software installed, disable that.
And try turning off the Firewall in System Preferences > Security & Privacy