For Windows based OS, use a 2 prong approach AV and FW. Avast is one of the better AVs(based on recent AV shootouts), the MS FW component is adequate.
For your browser adjust your security settings appropriately to limit popups and script execution.
More important is be careful of what sites you visit and things you should not be clicking (like click here for a free AV scan). Any Torrent files you download (.EXE or zipped) may contain virus, especially zipped because if it's encrypted, AV won't be able to detect until you try to run it.
Ensure you security software is updated regularly. Many Malware writers design their applications to defeat popular security software.
What you should do is reload the OS image once a year (max of 2 years). It's like getting a new PC.