Thanks very much for the responses, and I've now got it to an acceptable level of functionality.
DavidR - You called it correctly, as the lengthy scan parameters were set for both 'thorough' and 'archive' checking, though I must have done that under the presumption such a configuration would take less time. I reset the toggle to 'standard' and unchecked the 'archive' box, and it ran through this weekend in just under 3 hours. Since I normally set the various scans on my different machines to run overnight during the weekend, I'll kick the avast! scan off on Saturday evening.
Tarq57 - I previously had NAV set up on this laptop before switching to avast! about 6 months ago, but took much care to remove it completely before installing avast! (i.e., uninstall from Control Panel, run Norton Removal Tool, use Windows Explorer to track down and delete any drive folders with "Symantec" or "Norton" in their name reference, and finally edit the registry to track down and delete all subkeys if appropriate or subkey values which had a "Symantec" or "Norton" reference).
Thanks again for the feedback and advice.