I had this laptop that kept inserting letters when I tried to login. It would stick a bunch of extra letters in after every key I typed. Seemed like if I waited long enough then it would stop and I could login. Or if I rebooted six times. Sometimes it seemed like a certain combination of characters hit into the keyboard out of frustration would work but the combination was inconsistent so that was most likely a coincidence.
Needless to say, logging into my computer with these extra characters spewing out of my keyboard was a total waste of time. I was so annoyed I went and bought a new laptop.
Then wandering around the random Internet I happened across a white paper by Microsoft talking about messing up key loggers by inserting additional random characters as someone typed passwords into text boxes. AHA.
I had McAfee software installed and although I tried to uninstall it something was completely hosed on my machine and it didn't work right. There were still random services floating around that I would mostly ignore.
I went to start/run and typed msconfig to pull up start up programs and sure enough there were some McAfee services in there yet. I killed them all and removed them from automatic start up on my machine. Yep. That stopped the random characters from messing up my login.
This is not the first time I have had problems with McAfee and the feeling like it is more a foot in the door for hackers through their update process rather than a piece of security software. This feeling was justified in the case where their "enterprise" software had a serious vulnerability doing just such a thing. See other comments in past entries here.
And if you're having problems with extra characters appearing and getting in your way...msconfig and stop the McAfee services from auto start up.