I also use the Avast Boot Time scan on the odd occasion even if I am unaware of a problem, obviously it's possible to have certain virus's without even knowing it, so the odd Boot Time scan does not do any harm, except I will admit seems to take forever, but then again what price do you put on peace of mind?
As for the original post, you could also try at least two of the on-line scans to see if they are more up-to-date at removing the virus.