For Windows:
Start-> Run
Enter "cmd" and hit enter.
post the output of these commands:
ping xenforo.com
tracert xenforo.com
ping www.xenforo.com
tracert www.xenforo.com
ping
184.173.238.122
tracert
184.173.238.122
For Mac, you can use Network Utilities to ping and traceroute (remember to select 4 results for ping).