Vista may display the words “Test Mode” in the four corners of the screen if you install a driver that’s not signed correctly. I was able to remove the words from the screen by following instructions that I found here.

1. Push the Windows key. This will open the program search bar.
2. Type

   cmd

   This is the name of the command-line prompt.

3. Type CTRL+SHIFT+Enter. This will run the command line prompt with elevated (administrator) privileges. Elevated privileges are required to make the change.
4. Vista will prompt you to continue.
5. Type

   bcdedit.exe /set testsigning off

   and press return.

6. You should see the words The operation completed successfully.
7. Reboot Vista.

What Does This Do?

The BCDEdit program edits boot configuration data. The TESTSIGNING option controls how Vista handles test-signed kernel-mode drivers. More information on BCDEdit can be found at the BCDEdit FAQ.

Warning

It’s possible that this does something nasty. I have not noticed any ill effects to date.