User Accounts

I set up 4 user accounts in Vista. My son went in and made himself an
administrator (can't remember the reason). Soon after, his became the only
account we could access. The others didn't open; just a black screen. I was
able to open mine, finally (can't remember how), but yesterday, it stopped
working again. I tried creating new accounts, deleting old ones, scanning and
repairing, etc., but all I get when I open the account is a blue screen. Why
can't I open my old user accounts or make new ones????
--
CharlieS