Is your AVG updated every single day on one computer and not the other? could it be that you do not use one computer as much as the other one, therefore when you do go to update the anti-virus it takes longer because it has to download more definitions to update a more 'older' AVG than your main PC that takes 30 seconds?
What about hardware, are both PC's similar spec? Or is the PC that takes 30 seconds a much faster computer therefore installation is quicker?