Actually this has only happened to me when I'm low on resources, not physical RAM. Resources meaning GDI, USER objects etc; every windows o/s has these resource limits. Closing a few IE windows (of the many I had opened at that time) fixed the problem. I can't imagine why RAM would be a limiting factor, since that's what the paging file is for, unless some objects are required to reside in physical memory only (doesn't sound very probable, but I wouldn't know anyway).
