Gemma,
I would expect windows XP machine and a new update like service pack 3 has been installed (probably without you knowing) This added more security and may have changed the way you login.
Try this
1) Click on Start - Run
2) Type in the following command and click OK
control userpasswords2
3) Highlight the user you want to log into automatically, then uncheck the box for
"Users must enter a username and password to use this computer"
4) Click on Apply and you'll be asked to verify the username and password to log in automatically
5) Click OK and the next time you restart your computer, you'll automatically be logged in without having the classic prompt or Welcome Screen.
Hope this helps
Darren
October 2008