Test your computer for open ports
Broadbandreports / dslreports has a nice utility for scanning your IP address to check for open ports on your computer. Open TCP or UDP ports can be used by an evildoer to crash your machine, propagate a virus, guess your password and log in, etc.
A basic firewall, such as the ones built into practically all routers and OSes today, will fix most of these problems. This test is good to run if you don’t know much about networking or if you, like me, have to do a lot of configuration edits on the router and are not sure if the thing is still secure or not.