Ok, that I did not know.
I just looked into that, and its kindof sad. It works for 8 but not 8.1.
Even though its not listed, I am hoping that DDS still works for 8.1...
So, replacing Combofix with DDS, please uninstall Avast, and after a restart, download DDS and follow the instructions. All this program does is capture more information about your computer, but it will give us a final answer to at least eleminate what is not wrong, and possibly tell us what is.
DDS can be found here: http://www.bleepingcomputer.com/download/dds/
Please follow the instructions that DDS gives you, and once completed, download the avast installer from avast.com and install avast. You will find some great changes from the 2014 version anyway, so its worth the time.
As for combofix not working, I wasnt really sure that it would tell us anything different anyway so its no big deal at this time.
After you have reinstalled Avast, we can start the cleaning process.