Well, the obvious answer is to use the correct password for whatever the username is...
Does this happen when you're trying to sign on to Windows, or does it mysteriously pop up on its own after you've already signed on and are doing something unrelated?
If it's the latter, you can open up Task Manager (Ctrl+Shift+Esc) and click the applications tab to see a list of open tasks. Right-click on the task whose name matches the one in the title bar of the message and select "Go to Process" to find out which process is issuing the message. That might help explain what's going on.
It could also be a program that starts when you start the computer. I know I used to get a similar problem. I had to go into msconfig and disable the windows messenger from loading automatically. That was causing the annoying dialog box.
Its a good thing to do anyway. Many programs can be disabled from starting when windows starts. I only have my virus and a few others essentials enabled.
Check out the MSKB for msconfig instruction.
I have a pop up window that says "Sign In, User name and Password do not match."
How do I eliminate that?
I can tell you where to start looking. Go to Start>Run>type in gpedit.msc. Look around its in there somewhere. wish I could get you closer but I'm fighting a Win 7 install