Two security methods are a necessity. A active, runs automatically anti virus and a Firewall. They start up and run every time you boot your computer and stay running until you Shut Down your computer.That your first line (layer) of defense in most cases. I use MSE and the built in Window 7 Firewall.
Microsoft Security Essentials - Free Antivirus for Windows
A good second layer of defense many use are called (on demand) programs. Meaning they do nothing until you tell them to. No automatic anything. Super Anti Spyware and Malwarebytes Anti Malware are both great programs IMO.
Both SAS and MAM have paid for versions that do run active. If you desire.
My next layer of protection is a great little free program, Filehippo Up Date Checker.
It will check for updates of many programs on your computer such as Adobe, Jave, WLM, Firefox and many others. Most updates for programs like these have several security up dates included. This I also use on demand and I check at least once a day.
FileHippo.com Update Checker - FileHippo.com
All these programs I use.
My MAM is the Pro version and runs all the time on my computer is booted. All these programs work well with Windows 7 in my experience.
No one programs does it all that is why I recommend a layered protection plan.
The last program is me. I try and keep myself tuned up and updated so I don't do dumb things.
What ever programs you select after installing and updating them run complete scans. If you run into problems get back to us, we are always home.