yes a firewall maybe the problem. also your laptop could be infected by some virus or spyware or adware which is preventing your antivirus from updating.
i would suggest that you get a good antivirus (with updated definitions) on another medium like cd and scan your entire machine. Also scan for spyware and adware programs.
ad aware is a good software.
also try this ( i use it too on windows.)==>
spyware terminator
hope this helps. c ya!!