Were are you using it from, e.g. from windows safe mode or normal mode ?
What errors are displayed ?
Did you first try to uninstall avast from add remove programs and if so again any errors, etc. ?
If you either didn't uninstall first or couldn't uninstall using ad remove programs then avast and its self-defence module would block the uninstall utility (unless used from safe mode as avast doesn't run in safe mode).
Why did you need to uninstall avast if you don't mind my asking ?