I use a virus scanner on my Mac (better safe than sorry).
I heard issues about Norton (any product) running on newer versions of OS X.
McAfee as a scanner called Virex but you can get the free ClamAV.
So to answer your question, get the FREE ClamAV and stay protected.
I believe a normal value for MTU is 1492 or you could round off to 1500.
Check your Airport Express settings and make sure it is setup to accept 802.11g/b wireless and not just 802.11b.
The "b" band being slower. I also believe that even if "g" is selected once you reach a certain distance it...
To change the MTU value following the direction in the Help Support Articles titles"Mac OS X 10.4: How to change the MTU for troubleshooting purposes."
Also, you should try disabling support for IPv6.
Click the "Configure IPv6" button under the TCP/IP tab in Network Preferences.
Change from...