Windows vista log in prob.

Hello, ok I have a huge prob. I have never seen this before so I hope someone has...
Ok, I went to log on to my computer and when I get to the windows log in I put in my password and it seems to be loading but all it does is the log in screen will blink and come back on like it was before I clicked on the user name to put in the password. I have 2 diff user account on this computer family and admin the family does not have a pasword but the same thing happens when I clink on the icon to log in. I have alot of exp with computers but I have never seen this before. anyone fave any ideas I'm going crazy over hear!lol
O/S- vista home premium
my computer is cust made by me so I know all the info about it . so anything that anyone needs to know about it I should be able to explain.
thanks.