Hello northern! I will be happy to assist you in this what could be a severe case. Your avast detects a MBR virus.
MBR is the Master Boot Record. This contains all info on which OS to boot, the hard drive it is installed in, stuff like that.
Avast may not be removing it, but quarantining it so it will not delete anything. I would like you to do the following:
Please download Malware-Bytes Anti Malware: http://dl.betanews.com/malwarebytes/mbam-setup.exe
Install it and update it, then, run a full scan, and post back the log file when the scan is completed