removal of malware

I have avast premium. My google browser has been hijacked by Volstran malware. I performed a scan and the scan did not pick up the Volstran. How do I remove this malware- and how did it get there to begin with? since avast was already on the computer?

This needs further analysis by a malware removal specialist:
Go to this topic http://forum.avast.com/index.php?topic=53253.0 for information on Logs to assist in cleaning malware. Use the information about getting and using the tools. Start your own new topic in the Viruses and Worms forum https://forum.avast.com/index.php?board=4.0 and attach the logs there, not in the LOGS topic.