- Hack the registry to turn it off:
Usual rule applies, if you don't know what this means, don't do it.
You will probably make your computer break, and then you will be sad.
Windows 8 users (if there are any), can change it via group policy:
gpedit.msc -> Computer Configuration -> Administrative Templates -> Windows Components -> Credential User Interface -> set the "Do not display the password reveal button" to enabled.