Don't know the specification of your computer and operating system, but usually this a sign that you need more real memory and that your hard disk is getting a little full.
If you're using Windows XP it can just about run in 128mb but to do that it has to keep swapping memory to the hard disk as virtual memory, which slows everything down. With 256mb the whole of Windows itself can fit into memory, with a little room left for programs, but if you are running several programs there may still be some virtual memory use. Usually this is handled unobtrusively, apart from a drop in performance, but if the hard disk is getting full or needs defragmenting Windows occasionally has to stop to sort things out, which is when you get this error message.
One of the reasons why I'm planning to avoid Windows Vista is that it needs a minimum of 512mb to run, a gigabyte to work without frequent memory swaps. "Vista Ready" only means that a PC can run Vista, not that it does it well...
