Hi tlee,

Try the steps below and see how those work for you.

1 - Download the new av program but do not install it.

2 - Download the uninstall tool for the present av but do not use it.

3 - Disconnect from the Internet.

4 - Uninstall the present av using Add/remove programs (XP) or Programs (W7). I am not sure exactly how this is named in Vista.

*5 - Restart the computer.

6 - Use the uninstall tool downloaded in step #2.

*7 - Restart the computer.

8 - Install the new av program.

*9 - Restart the computer.

10 - Reconnect to the Internet.

11 - Update the new av program.

*The multiple restarts may seem excessive but it is to insure that the actions taken are cleared and updated in the OS and registry before taking the next steps.
Also, if the computer is set to automatically connect to the Internet on start-up, disconnecting from the Internet will be needed after each restart except in step #9.