Is it a fresh install of OS? I assume it is. Uninstall Avira/Antivir, restart pc, open a web browser, still crashes? Update your OS & software including antivirus. I know antivir is v8 and avira is v9. Some prefer the old one. Always use the new one.
If after software updates, web browser still crashes. Write this to a disk & boot it up for 8 passes: (may take a night)