Why does my computer say it's low on memory when no apps are running?

There are always things "running" even when there aren't any windows visible on the desktop. Virus scanners, spyware monitors, software updaters. I'm gonna assume you're using windows... look at the taskbar on the bottom right. All those little icons represent programs that are running. I've seen people with 20, 30, 40 items down there.

IF you use ctrl-alt-delete and look in your task manager, you'll see what's using up all your memory. Task manager isn't necesarily the place to shut the offending programs down; you might have to uninstall something.

Sometimes memory concerns can only be remedied by a reboot, but if your concerns are persistent, it's time to uninstall something.