How to disable Windows Xp Balloon Tips?
Note that making incorrectly changes to the registry may damage your system; please back up any valued data on your computer.
1) Go to Start menu > Run > and type regedit and click OK.
2) Find following address: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced
3) In left panel select and right click on Advanced key, create a new DWORD and name “EnableBalloonTips”.
4) Double click on new DWORD and change value to “0” (Hexadecimal).
5) Exit from Registry Editor and restart your Computer.

Soleimanian

The eliminate the risk of modifing registry, I personally recommend TweakUI which is come with Microsoft Power Toy.
It allow you to complete this task by just one click.
(a screen shoot is provided for who does not know what TweakUI is)

As you can see from the screen shoot, TweakUI can do many little things easily, such as remove the arrow from shurtcut.
you can easily undo the changes you made.
It is better than to modify the system's registry, and more safer

Hmm... Im going to try that program when I get home, Im always nervous when I do changes in the registry, because Im afraid I might need that feature and cant remember how to get it back, its a pain when that happens. Althought it hasnt happened yet, im just kinda paranoid.