Every now and then someone will report that they are having problems launching more than 10 or so virtual machines under Virtual Server.  Inevitably it turns out that they have enabled the /3GB boot option (http://msdn.microsoft.com/library/default.asp?url=/library/en-us/ddtools/hh/ddtools/BootIni_de16d3ec-c437-4628-805f-8945ea598a92.xml.asp).  This option is usually recommended for applications that need lots of memory - it increases the amount of user-mode virtual memory available from 2GB to 3GB.  However it does this by reducing the amount of kernel-mode virtual memory from 2GB to 1GB.

And unlike most applications - we use a lot of kernel-mode virtual memory - in fact even without the /3GB switch users are most likely to run out of kernel-mode virtual memory before running out of user-mode virtual memory.