I get taht too.....i recon it's viruses....took me a while to clean them out before it turns good again....anyways, if you use the net to play games, then i recommend that you close all browsers you left open, because it increases the amount of memory needed to store temporary files from the net.....which can both slow your browsing speed and computer....
One way to ease a MB or two from your RAM is to disable the windows xp visual style...
1)Right-click on "My computer" and click properties
2)CLick on "Advance"
3)Look for "Performance" and click on "settings" then another window will appear, so click"visual Effects"
5)Then un-check "Use visual styles on windows and buttons" and "Show shadows under mouse button"
This is just tiny things that i recommend you to do....how ever if you were to disable the visual style feature, then your screen will have the style of windows 2000 which i think that there is not much of a difference from a windows xp visual style.
Thats all i will tell you to do....cause that is what im familiar with...and it would be time consuming to introduce you to what the rest of the items on the list.