I have Comcast also and they offer the McAfee suite and I tried it and disabled and then uninstalled all things McAfee after a little investigating of why my system was running so slow.
Don't count on McAfee for help with performance issues! Just disable all their stuff for a while and see how it goes, you can turn it back on or reinstall it later for free if you want.
I do have other software for protection from malicious software, but I will not use McAfee - too slow, long updates, always doing "something" and it never found or fixed a darn thing (ever) that other softwares pick up.
Right now, I like the free ones I can run when I feel like it and although there is no real time protection, that is okay for me, and may not be okay for everybody.
Currently, I just use these two regularly and the built in Windows firewall:
Malwarebytes (MBAM): http://malwarebytes.org/
SUPERAntiSpyware: (SAS): http://www.superantispyware.com/
If you try them, update after downloading and these can be uninstalled later if desired.