Problem: 
AT&T Internet Security Suite cannot be uninstalled with Windows Add/Remove Program or the Uninstall AT&T Internet Security Suite option. The cleanup utility couldn't remove AT&T Internet Security Suite either.

Resolution:
To resolve this issue, you must use a special utility, Windows Install Clean Up, developed by Microsoft.

1. Download the utility by clicking msicuu2.exe in the Downloads section below.
2. Double-click the downloaded file to start the installation.
3. Once the installation is complete, click Start point to  Programs and select Windows Install Clean Up.
4. A list of programs will appear.  AT&T Internet Security Suite should be on that list, possibly more than once.
5. For each entry of AT&T Internet Security Suite, select it and click Remove.

To learn more about the Windows Install Cleanup utility, please visit Microsoft's website:
http://support.microsoft.com/default.aspx?kbid=290301