DIY PC Servicing Article:
Remove disturbing Bubble Tips on WinXP

You need to edit the Windows registry to remove all those disturbing bubble tips.

http://www.dougknox.com/xp/scripts_desc/xp_balloontips.htm
http://support.microsoft.com/default.aspx?scid=KB;en-us;307729&

If you don't want to edit the registry, get Tweakui powertoy for Windows XP. Under the "Taskbar and Start menu" node in the tree, you can enable/disable balloon tips.

TweakUI for XP:
http://download.microsoft.com/download/whistler/Install/2/WXP/EN-US/TweakUiPowertoySetup.exe

For XP w/SP1:
http://www.microsoft.com/windowsxp/pro/downloads/powertoys.asp