One of the most annoying pop up balloons I see when working on other peoples computers is the Desktop Cleanup Wizard in XP.  I really see no reason for this feature to be enabled by default.  Here is how to disable the annoyance.

Right click any open area on your desktop and select Properties and click the Customize Desktop button.

In the Desktop Items window uncheck the box next to Run Desktop Cleanup Wizard every 60 days.