If you are still looking for an app to customize the Windows System Tray clock, it's really a good idea to have a look at Tclock.
The application has been specifically developed with this goal in mind, but it also offers some other utilities, such as an alarm clock and a timer.
Exclusively addressed to Windows XP users, Tclock provides a very simple customization interface, allowing you to change colors and fonts with just a few clicks.
You're thus allowed to pick background, text, font, size and adjust its position, but also height, width and line height.
Further customization is of course possible by defining the data to be shown, such as year, month, date, day, seconds, minutes and hours, with a special field to configure the format.
As said, the package also includes an alarm tool that lets you schedule alarms and play a sound at a specific time.
As you can see, Tclock comes with pretty basic utilities, which means it should work pretty smooth, without affecting the overall performance of the computer. And truth is, the application is far from being a resource hog, but the fact that it's compatible only with Windows XP is indeed a major setback.
All in all however, Tclock is a decent solution that could allow Windows XP users customize the System Tray clock quick and easy.
It provides a reasonable amount of customization options and although most of the users expect skin support and much more appealing features, it's still a simple way to change the old-fashioned look of the clock.