I'd like to have a program with the following functionality: I could click on a tray icon and have a popup appear with some notes, and then I could click again and it'd go away.
Is there such a program for Windows XP?
I use ActionOutline for this. It can sit in the tray and open with a click on the tray icon or with a shortcut (Win-A). It's keyboard friendly too, so I can quickly make a note and minimize it with Esc and never touch the mouse. It's using 12MB of RAM on my machine right now.
There's a free version that is limited to 7 items per tree item. The paid version starts out at $49.95, but they occasionally have discounts. I paid $34.95 IIRC.
I use OneNote a lot, but for certain things, like writing documentation for computer tasks, I prefer to use The Guide.
It does not run with a system tray icon, but you can assign a keyboard shortcut to its Start Menu shortcut, which should give you the sort of functionality you are looking for. In its options, you can also assign the ESC key to close the program.