If your computer is setup to give a list of account names, where you click one and enter the password, the administrator account is by default not listed.
The default name for the account is 'administrator' (not case sensitive).
To access it, at the 'welcome' screen (the one that lists the accounts on the computer), hit the CTRL+ALT+DEL sequence twice and you should be prompted for a username and password. At this point, enter 'administrator' for the account name and the current password for the account.
Another thought is that if you are asked for an account and a password, you can just overwrite whatever is listed there. The default is to list the last account that was used, but that doesn't mean you can't change it - in fact that IS how you switch accounts if no list is provided to select from.
The idea being that not listing account names means you need two pieces of information to logon - the username and the password rather than just the password. This config is how networked computers operate in general for a windows environment.
Possible roadblocks:
1.) The administrator account has been renamed to something else like 'baba-yaga' or 'admin' or whatever.
2.) The password for the account has been changed.
If this is your own personal computer, then there are plenty of freeware utilities you can download from the internet that will work as long as you can boot the system to either CD rom or Floppy disk.
Look for said utilities and you should be able to find one that will show you all account names and to reset the Administrator account password.
Warning: I've had to use utilities such as these many times. DO NOT use them on a domain controller or you will mess everything up. For most workstations (IE - personal computers) you should be fine.
Warning: There are applicable laws to consider - if this is not your computer, you are sure to be breaking those laws. Also, if this is a corporate computer for your employer, you very well may be breaking policy that could get you fired.