I also find it hard to believe a computer would go to sleep then not accept your password, you could do as the person above me stated, the only thing to make it possible is, I believe win 7 has the option to set your password to be changed every so many days, not sure if it's set as default or not, but that's the only way besides the obvious way for the computer to not accept your password.
Only advice I'd offer is bring it to a computer tech shop. I don't know if they check to see if you own the computer or not though.