After on-demand, in-depth scans with all options on WinXP and Win2K and 15 hr real-time elapsed on both I can report that resource use (memory, CPU) appears to be outstanding!!!! Real-time is rock-steady between 28-32 MB. On-demand adds 10-12 MB with archive spikes adding another 20 MB. Win2K will saturate CPU usage, but throttles if other processes demand. This is with 551.