I am running a fairly big frequency study with about 2 million nodes on a 64 bit Xeon system with 32GB of RAM and Windows XP 64 (which may be the problem) and SW 2011 SP 3.1. I let it run overnight (about 15 hours) and when I checked in the morning I had the message that virtual memory was being used and to expect "delays". Checking the performance tab in Task Manager I noticed that the peak commit charge had exceeded the physical memory at some point but was not presently exceeding the limit. Checking the harddisk I noticed the the entire 35GB had been dumped to the harddisk and almost completely filled the C drive, bringing the system to a crawl. As soon as I terminated the simulation, the 35GB disappeared from the harddrive and the systtem returned to normal speed (after a re-boot). I did a search for topics related tto this issue on this forum and found similar issues in earlier versions of SW, has anyone experienced this with a similar system? Does Windows 7 handle memory better?