Just had this problem on Windows 8.1
After determining that the uninstall process was going nowhere, I ran Task Manager. Under Processes saw instup.exe and another (Avast) wsc_proxy.exe running. Right-clicked on the wsc_proxy.exe and selected End Process Tree. This process terminated, several others started and terminated (quite quickly) Repeated End Process Tree on subsequent Avast uninstaller executables until prompted to restart. Avast is now uninstalled