It might be that the free trial does not include updates – but I doubt it, since such a strategy would leave your PC open to new viruses.
It looks like for some reason your PC is not automatically updating with the Norton definitions.
You could try contacting Norton, and ask for their help – you might be lucky, especially if they take the view that you will purchase an alternative anti-virus program if they cannot sort your problem.
http://www.symantec-n...ct-us.aspx?lang=en-GB
Besides asking Norton for assistance – I would start looking to install an up to date anti-virus program, given the potential harm a virus could inflict on your PC.