Never keep two active virus protection programs on your computer. Virus protection systems are designed to scan aggressively to protect your investment. Having two virus protection programs running at the same time will bog down your system as they "fight" for the right to clean your files.
Unfortunately commercial virus protection isn't cheap and has an annual renewal fee. Though not a effective as full package, AVG's free version provides basic virus protection.
You're smart enough to know that few things in life are really FREE. Use common sense when installing "free" software downloaded from the Internet. Take your time and read the install prompts. Uncheck any additional programs, toolbars, games, etc. included in the package that you don't specifically want. Charging other companies to Include those "extras" with their product is how developers of good tools make money to feed their families so they can give away the good stuff.