I set up Windows 7 with a couple of standard accounts, and an administrator account (called Odin). I can log in to Odin just fine from the Welcome screen. While logged into Odin I can do anything that requires administrator privileges without a password: just hit Yes in the User Account Control dialog.
If I am logged into one of the other accounts and I do something that requires elevation (e.g. most installers, and some control panel functionality), then Windows presents me with a User Account Control dialog asking "Do you want to allow the following program to make changes to this computer? To continue, type an administrator password, and then click Yes."
The account shown in this dialog is Odin, so I enter Odin's password. But Windows redisplays the dialog with the message "Logon failure: unknown user name or bad password." This always happens, and has done so for many months, probably since I first got the computer.
Why does Odin's password work from the Welcome screen, but not when elevating? Please note that I am not asking how to recover Odin's password. I remember the password I originally set for Odin, and it works as it should from the Welcome screen, but is not recognized when elevating. The password has no funny characters, just letters and digits.
Update: Because of a hardware problem, I had to wipe and re-install this machine. At first I did not experience the problem described above. After I installed Windows Live Essentials (for Family Safety), the problem described above recurred. Although the account I was logged into was not controlled by Family Safety (as it was my own account), I still experienced the problem of the administrator's password not being recognized. When I uninstalled Live Essentials, however, the problem disappeared.