Some of the AV products that advertise low resource usage seem to have differing execution profiles, in terms of how much physical vs virtual memory is consumed. It is easy to understand physical memory usage, but for me virtual memory (VM) is a bit less clear. I have noted that the current versions from ESET (NOS32 and Smart Security) grab a decent chunk of RAM (35-40 MB in my case), and hold on to it, but the numbers do not fluctuate much. Other products seem to use less RAM (10-20MB), but at times I see widely varying numbers in the Virtual Memory section of SysInternals Process Explorer. In one case, on a Win XP machine with 512MB of RAM, "Peak Private Bytes" was over 800MB, the "Virtual Size" was over 1GB, and "Page Faults" had reached 1,5 million. During this time, the system was very slow to respond, and appeared to be performing a lot of paging to disk. In these two examples, the software using more physical RAM generated fewer perceived performance problems, than the one using less physical but more virtual memory. Are large VM numbers indicative of a software problem? Is there a general rule of estimating what VM values are reasonable given the amount of physical RAM in a machine?