I belive, but im not too sure as im still new to xp, that it means your swap file is full. But your right on key, when your ram fills up, it dumps to your swap file. I usualy set mine to 300mb.
Once swap file is full (virtual memory) the only way to clear it is to reboot that i know of. Thats why i run a system monitor that lets me know whats going on.
The one i use you can get at
www.mdev.tk seems to work good on xp AND you can shrink it up so its about 2/3 the size of your mouse pointer.
For the direct download link click
Here
To get the temperature thing to work if you have a mobo that supports it you need to download the mbm addon. You can see a picture of mine on the screen shot i posted
Here at the bottom right of the screen.