What did you or someone else change? This is how to enable smart card logon, http://support.microsoft.com/kb/281245,
someone set that up on the computer, you need to contact them to change it.
Some more info http://support.microsoft.com/kb/834875
You can start the PC in Safe Mode and undo the smart-card settings (Hit F8 when the PC is booting right after the BIOS screens, and select Safe Mode).