For some reason I’m always looking for this. This applies to Windows 7. Not sure about other versions.
You must run regedit as an administrator.
To disable these balloon messages follow the steps below:
1. Open the Registry Editor by clicking on Start -> Run and running REGEDIT.
2. Navigate to the following location:HKEY_CURRENT_USER\Software\Microsoft Windows\CurrentVersion\Policies\Explorer
3. Right-click in the blank area of the right-hand pane of the Registry Editor and create a new DWORD Value by selecting New -> DWORD Value.
4. Name this new DWORD Value NoLowDiskSpaceChecks
5. Double-click on the new DWORD value and assign it a value of 1.
6. Close the Registry Editor and restart the computer.
This can also be applied via Group Policy.