As an avast user - So far the majority of reported cases in the avast forum have been on XP systems, so it does look that way. But that may not be 100% and there is a possibility that depending on the systems configuration and installed software there could be cases on other OSes, for a slightly different reason.
Given the huge number of differing system configurations out there, it is possible some other systems may be effected, but the ones seen here tend to be on XP.