When you manage a cluster, you need to be aware of how much virtual memory is available at any given time. The procedures for monitoring virtual-memory usage are well documented on the Microsoft Web site. The following articles offer a good starting point:
The Microsoft article "XADM: How to Gather Data to Troubleshoot Exchange Virtual Memory Issues" (http://support.microsoft.com/?kbid=317411) explains the steps you can take to capture system information to assist troubleshooting if virtual-memory problems occur.
The Microsoft article "Exchange Server 2003 and Virtual Memory Fragmentation" (http://support.microsoft.com/?kbid=822180) discusses some of the changes Microsoft has made in controlling memory fragmentation in Exchange 2003.
Since Exchange 2000 Server Service Pack 1 (SP1), Exchange has incorporated a set of performance monitor counters specifically designed to monitor virtual-memory usage on a cluster. The Microsoft article "XADM: Monitoring for Exchange 2000 Memory Fragmentation" (http://support.microsoft.com/?kbid=296073) discusses the counters you can use.
If virtual-memory fragmentation problems arise, you might also need to monitor the virtual-memory usage of third-party products in your clusters. First, make sure that you have the latest version of all third-party products. Next, for each product, monitor the virtual memory with the product enabled, then disabled to see whether the product affects virtual-memory usage.