well you can either ad more ram, or right click on my computer and goto properties, under the advanced tab, theres a setting for performance, under the advanced tab again, you can change the size of your virtual memory. just set this higher

This was assuming your running XP, if not it should still be somewhere under the properties of my computer

basically virtual memory is space on the harddrive set aside so that when your using too much ram, it can use the harddrive space to supplement the ram, the only problem is that the harddrive is slower than the ram so you can run into issues with performance.

How much ram are you running? what processor? what programs are you running when you get this error?