Shame it's not a BIOS password, that would have been an easy fix. You should try JiK's blank password suggestion (just hit enter) on both user accounts. Another possibility is the "hidden" Administrator account. Usually this is not enabled, but it's possible that your PC supplier has enabled this account and set a password. You'd need to ask your supplier.

Finally - has anyone else been using your PC? You say it hadn't asked for your password until a few days ago. I wonder what might have caused that to change?