This tip turns off the pop-up reminders, for advanced users. For the
general users (Mom, Pop, you cousin Winnie), you should leave this
enabled, so that they can give you a call.
This reminder will still allow you to see that your drive space is
low, but without the balloon messages popping up, in the middle of your
work. Another thing you will miss is Windows' reminder that hard-drive
space has run low.
This can be done, by getting Microsoft's Power Toys, for power
users. You can go here
and download Microsoft's PowerToys. This is a set of essential
applications offered free from Microsoft. They are unsupported.
XP PowerToys is easier to install than previous versions and
resides in the program menu, rather than the control panel.

Once you have it installed, go to Taskbar and uncheck "Enable
Balloon Tips." This is the easy way to do it. And, it is only a 1MB
download.