Simple idea involving no extra apps (in other words, no extra $):
Add a note to the contact entry of every one of you customers:
08/25/2004
Formated hard drive
08/26/2004
Installed Windows XP
08/27/2004
Received payment (800USD)
Windows interface? Sure! Palm desktop.
Is it too limited? Well, it depends on your needs, of course.
But, it is easy to access and takes into account the fact that "text entry on the Palm is just not good enough to write more than a few lines!"