A virus scanner is provided in which a pool of pre-processor threads and a queue are interposed between the event filter and the pool of scanner threads. The pre-processor threads perform operations that can be completed quickly to determine whether an object of a scan request needs to be scanned. The pre-processor threads gather characteristics about the scan requests and place them in the queue in a priority order based on those characteristics. The scanner threads select a scan request from the queue based on the priority order. Alternatively, the scan request is selected based on the scan request's characteristics as compared to the characteristics of the scan requests whose objects are currently being scanned by other scanner threads in the pool.