When you first turn on your computer
Goto run, cmd
Netstat -a
there shouldn't be anything there that doesn't look 'right'. Connected to other computers on your network is fine, also is being connected to them via port 0.
If you see some remote IP address ie 64.xxx.xxx.xxx:31337 connected to port # 31337 or some other suspicious port (search google for port list) you can even check every connection, there should not be many, for anything out of the ordinary.