Microsoft is trying to do two things with Windows 8:

1.) Protect all new users by providing a full-fledged antivirus program from the get-go on first use. This is different from prior practice on new os installs on earlier versions of Windows.
2.) Avoiding issues with running two active resident antivirus programs at the same time by automatically disabling Windows Defender when it detects a third-party install.

Seems fair enough.

So, yes, you will have to uninstall avast! in order to run Windows Defender on W 8. If you wish to do so, then use this tool, http://www.avast.com/uninstall-utility after uninstalling avast! normally, and reboot. You’ll need to check to see if WD is enabled and running as it should be on restart. If so, you’re good to go.