Log on Problem

When i use to start up my computer i didn't have to log on or anything, it just took me straight to desktop which is ok since everyone in my household uses the same account. We don't have a password either, but suddenly, whenever i turn my computer on half the time it says it's loading windows xp but it won't get past that. i have to keep restarting my computer hoping that this time it'll actually get past that. but when it does, a logon screen is in the background with the username administrator and there's a password in it but the window infront of it says that windows couldn't sign me in because the username or password is incorrect. I click ok, and it takes me to the other logon screen where I can choose an account (but we only have just that one). Is there something i can do about this?

Use this tool to manage autologons:

http://www.sysinternals.com/Files/Autologon.zip (Autologon)

I'd suggest you run a thorough disk check and scan for malware as well.