Given that it works fine on a different computer I would look at and compare service pack levels of the Outlook application, com add-ins, service pack levels of the OS etc. On the problem computer search for and delete the existing ost and let outlook create a new one.
With Outlook open, go to Tools > Options > Mail Setup. Check “Send immediately when connected”.
Click on the Send/Receive button on Mail Setup General tab. Make sure both options “Include this group in Send/Receive” are checked. Make sure that your account is included in the Group to send.
Just another thing to check. Does this happen when your client starts Outlook in safe mode?
Click the Start button and type this in the Open box field. Please add the quotes.
“C:\Program Files\Microsoft Office\OFFICE11\Outlook.exe” /safe
If that works check your addins:
In Outlook click Tools ~ Options ~ Other ~ Advanced Options ~ Addin Manager.
Uncheck all and restart Outlook. If things work normal, check each addin one by one and restart to see which one is causing the problem.