Troubleshoot slow connections

All,

I have just upgraded to FTTC (Fibre to the Cabinet) running Untangle on ESXi 4.1 and since upgrading the internet connection has not increased at all, my vDSL modem in front states I get 62Mbps but how do I rule out Untangle as my ISP is stating the issue is because of a software based firewall (i know!!)..

How do I prove them wrong?

Thanks

Keith