I use and recommend Zone Alarm, available free at Zonelabs.com
It makes all the ports, ie, connections points into your computer,
invisible to port-scanning hackers on the web. It also has other
features which allow you to control all access to the web by programs
residing on your computer. It is a very useful program which is easy
to use. Used in conjuction with a good anti-virus program to scan
your system it affords good protection from all but the most
sophistocated hackers, nasties, etc. Zonelabs also sells a PRO
version for $49 bucks, but I haven't needed it yet. Need I add that
you should never open any attachment which has an file extension with
which you are unfamiliar? My policy is, delete first, ask questions
later.
Rich