if you have 128mb of ram,
and you have 512mb of programs running, windows uses part of your hard drive, known as the page file or swap file, to act as ram for programs in the background.
usually, this lot on your hd is double the physical ram.... so in this case, you have 128 on the ram chip, and 384 on the hard drive.
when you have a lot of stuff open, you notice your hard drive acts up-- especially when you switch to a program you haven't used recently (windows puts the active progs on the ram, and the background ones on the hard drive)
now, say your page file was only set to 256, not 384. Bam, there's you're error, and windows should increase your paging file to accomodate the data.
nothing is wrong... it just means that your computer is a piece of shit that needs more ram for what you're doing with it