When I try to log into Yahoo Messenger, I get the first letter, but after that numbers in sequence appear?

When I type in the log in box, the first character is fine, but after that only numbers appear in the box, and increment sequentially. I have tried uninstalling and reinstalling messenger, but that didn't fix it. This only happens on yahoo messenger. I've read other posts with similar problems, and followed the suggestions. No change, still having the problem. Can anyone help?