Windows Vista Problem

Basically I tried to use my computer today to find that when I loaded it up, it asked me which operating system I wanted to use (it hasn't done this before). I selected Vista and it loaded up to the welcome screen where no log-in box appeared.
I rebooted and the same happened again, so I rebooted for a second time. This time it worked and the box appeared but I'm a little concerned about what could have caused this, and is there a fix for it?
Cheers :D

Well if it doesn't happen again why fix it? :P

Just wait to see what happens next time.