McAfee is terrible and DOES clog your PC down.
I don't agree with Project one bit.
Avast is free and is excellent. There's also a version you can buy which gives you more features. Used it for donkey's years when on a PC after it was recommended to me by a mate in IT (he's actually head of IT for a major company).
http://www.which.co.uk/technology/s.../avast--free-antivirus--2014-/customer-views/
http://download.cnet.com/Avast-Free-Antivirus-2014/3000-2239_4-10019223.html
Btw - try seeing the light and getting a MAC. Bye bye malware and viruses. Yes, both can still be a threat but it's easier to protect and there are less out there - a lot less. Used one for 5 years straight without replacing a bit of hardware. An outstanding bit of kit that's built to last. Anything running windows has a deliberate built-in shelf life and I vowed I will never use one again. My wife has gone through 3 windows laptops in the same period.
That said, a friend is a lecturer in IT at Swansea Uni and hates MACS but she's real old school bless her. Speak as you find I guess.