Real-time protection
Avast Free is a good antivirus ,however it's important to know that most infections strongly rely on
social engineering, basically you'll need to run them so that they can work,so if you practice a smart online behavior then you should really decrees your chances of getting infected. You can read some easy tips on how to avoid infections here: http://malwaretips.com/Thread-5-easy-tips-to-avoid-infections
Browser protection
What add-ons have you installed on your browser?And which one do you use most often?
On demand scanners
You should
always upload all your downloads on virustotal.com and perform a scan with your on-demand to be checked for malware!
VTUploader (Free) -
link
To upload a file to VirusTotal, you can visit the main analysis site, click the Browse button to select a file from your hard drive, and then click the Send file button. You can make this process even easier with the free VirusTotal Uploader utility. After installing it, you can simply right-click any file under 20MB and choose "VirusTotal" from the Send To Windows menu. The scan results will display in your browser as usual.
Hitman Pro (Trial start when you actually remove infected objects.) -
link
An on-demand scanner using multiple anti-malware engines and cloud technology. It offers unlimited free scanning but once you use it to remove detected malware it switches to a 30-day trial version. I recommend using it after you've scanned your hard-drive with the other products you have installed
Virtualization:
Even if you submitted a file to virustotal.com and it said that it's clean , you'll need to run it in virtual environment because sometimes infection can be so new that security vendors don't have signs for it.
Virtualization software will allow you to browse the web or run another application in a completely safe environment. This is especially useful when visiting high-risk web sites, whether accidentally or deliberately, as the Web browser will be completely contained within the virtual enviroment, preventing any damage to your computer.
A sandbox can also be used to run any other applications which you think may be suspect - you can run the program inside the sandbox to determine whether or not it is safe while remaining completely protected against any malicious actions that it may try to carry out.
I strongly advise you to install Sandboxie and use it for when you're browsing the Internet or running shady/unknown programs(not that you should do that but...
) . Alternately you can try
BufferZone PRO (Free) , another great virtualization software.
Sandboxie (Free/Paid) -
link
Sandboxie runs your programs in an isolated space which prevents them from making permanent changes to other programs and data in your computer.
Always run suspicious of freshly downloaded files in a Sandboxie to verify that he download isn't compromised!Sandboxie will replicate perfectly your operating system so all the files should run without any problems in it.