Actually, the solution was unnecessarily complicated

If the providers were only stopped, the following is enough to start them:
1. Click on the avast! icon in the system tray - the "On-Access Scanner" window opens.
2. Click "Details...", if not already expanded
3. Select the stopped provider and click "Start" button
The same window, asking if you want to persist the change, will appear. If you answer Yes, the provider will be automatically started again.