Here's a post from the Windows Server Performance Team:
An Overview of Troubleshooting Memory Issues
The post discusses the following issues:
- Physical RAM shortage
- Memory leak situations
- Poor pagefile performance
There are performance counters given to be monitored, as well as a discussion as to what they mean and what thresholds you should be worried about. Since database platforms tend to be heavy on memory usage, any DBA on Windows should keep this in his or her back pocket.