1. way
create file:

---
Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced]
"EnableBalloonTips"=dword:00000000


---

without lines. Save it as disable.reg
(All files, then name it disable.reg )

Start it.

-restart or log off from PC.

OR

2. way

-start/run/regedit
-navigate to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced
-right click in the right panel.
-New/DWORD Value
-name it "EnableBalloonTips", without quotes
-double clik it, set value to 0
-restart or log off from PC.


source: http://support.microsoft.com/kb/307729