If you are absolutely certain you didn't have an administrator password on the machine and hitting enter by itself doesn't bypass the screen... then it's time to condier the following:
If this is a computer that you received from your employer, the desktop support people sometimes will install an administrator password. If this is the case, they should have a record of the password.
If this is a computer you purchased from a reliable vendor such as Dell... then the same concept as above may apply.. and you should give them a call and see if they have your system configured as such.
If you purchased the computer secondhand then it is entirely possible that the prior owner had one set up and you will need to contact them for the password.
If none of that applies then you may have to purchase a pasword cracking program...
Windows Key from passware http://www.lostpassword.com/windows-howto.htm
There are also free programs like Ophcrack you can get for free which will attempt to crack a password. but if your system is down and/or you've been locked out from a virus or something then you may be stuck.
You could also attempt to simply reinstall windows. As long as you DO NOT choose to reformat the drive, windows will basically just install over itself. Saving your documents but you'll probably have to reinstall programs.