Your system can become unstable if it runs low on virtual memory, and there's a pretty good chance it will hang if it runs out of virtual memory. Older versions of Windows displayed one warning when they detected low virtual memory and another warning when the system ran out of virtual memory. However, in both cases, users were simply told to shut down some or all of their running programs. That often solved the problem, but shutting everything down is usually overkill because it's often the case that just one running program or process is causing the virtual memory shortage.

Vista takes this more subtle point of view into account with its new Windows Resource Exhaustion Detection and Resolution tool (RADAR), which is part of the Diagnostic Policy Service. This tool also monitors virtual memory and issues a warning when resources run low. However, RADAR also identifies which programs or processes are using the most virtual memory, and it includes a list of these resource hogs as part of the warning. This enables you to shut down just one or more of these offending processes to get your system in a more stable state.

Microsoft is also providing developers with programmatic access to the RADAR tool, thus enabling vendors to build resource exhaustion detection into their applications. When such a program detects that it is using excessive resources, or if it detects that the system as a whole is low on virtual memory, the program can free resources to improve overall system stability.