I use HouseCall (Trend) once in a while myself for a “second opinion” – especially if there’s suspicions of a false-positive from avast, or for that matter if I suspect something might be slipping past avast. Since they use different scan-engines, it’s highly unlikely they’d both report false positives or false negatives.
Don’t forget to pause your Standard Shield before using any of the online scanners, to avoid conflicts. And of course don’t forget to “resume” it afterwards.