How to Unlock PC, if i didn't set any Password for that?

Hi guys, I have a problem with my PC. My PC shows me logon screen and asking me for password. When i installed windowxp in my pc at that time i didn't set any password for login, i left it blank. But i don't know what happened, it still showing me logon screen after the window starts and asking for password. I left password as a blank and pressed on OK, then it getting login and logout immediately and showing me logon screen and asking for password again. I don't have softwares backup which i already installed on my PC, that's why i don't want to format my PC or reinstall Window XP. Plz help and tell me what should i need to do. I will very thankful for you guys.