TClockEx did not work right for me on XP (the appearance was buggy, all the functions were working properly however). Anyway, there's a very similar program called TClock which did work for me, so now I am using that. It can also do cool things to your taskbar, like customize your start button and other little things.
Unfortunately, TClock does not have a calendar function, which is annoying. So now I am looking for a calendar system tray program. The absolute best I have found is one called TrayDay. It's exactly like I want...just a calendar, nothing more. You can customize it's look, it's lightwieght, it can show 3 months at a time. However, it's $15 which is a bit much for something of that nature. Maybe someone can recommend a similar program? Or if it doesn't exist, maybe could do it as a new coding snack?