The problem with using Mcafee Security to monitor bandwidth is that every computer connected to your router either via ethernet cable or wireless needs to have the software. Some devices, like wifi phones, won't ever have Mcafee security. Your best bet to monitor your usage is to install Tomato firmware with SNMP support located here:
»www.avenard.org/wrt54-tomato/tom ··· -vpn.zip
(credit goes to a poster named Gherkin on the whirlpool forums: »forums.whirlpool.net.au/forum-re ··· 661.html
and then use the free version of PRTG to track bandwidth usage directly from the router. This is the most accurate way to make sure you count every last byte going over Comcast's network. This is how I've setup all my friends on Comcast and is your best bet to making sure you don't go over the limit.