Write shorter .asm programs!
...Or buy more memory.
Seriously, I've never seen that message from "nasmw" - "nasm16" (*NOT* recommended!) does it all the time. How large a file are you trying to assemble when this happens? Anything else running that could be using a lot of memory? Nasm is actually pretty "stingy" with its memory usage (so "nasm16" is possible - this will soon be dropped), so it *shouldn't* be happening! If the problem doesn't go away after a fresh reboot, post more details - you may have uncovered a bug...
Best,
Frank