First thing I did when I got my Windows 7 Home Premium Service Pack 1 was uninstall the Norton they offered. I installed Avast Free Home Edition using custom settings. I also use the older version of Spybot Search and Destroy and the Windows Firewall.
I've used various free and paid for programs at home and work. I've been happy with the above programs for the last 5 years. The problem with programs is even the paid for versions are not set for maximum protection by default.
As far as slowing a new computer down with software I've not had problems and mine only has 4 Gig RAM
AVG free isn't much good and we usually removed it from customers computers. Avast free for home use is a great product if you know how to reconfigure the settings for maxium coverage. Even purchased products need settings reconfigured for good coverage, but why pay if you if you can it free?