Recovering (or creating a new) admin account
Download and install a fresh copy of Craft from http://buildwithcraft.com somewhere (doesn't matter if it's the same computer your existing site is on or not).
During installation, remember the password you entered when setting up the user.
Open the database and look in the craft_users table. Copy what's in the password column for the user that the installer created over to the password column in the craft_users table for the site you're trying to recover.
If the locked, suspended, pending or archived columns have 1 in them, set them to 0.
If lockoutDate and/or lastInvalidLoginDate have values in them, delete them.
Login to the site you're trying to recover with the password you created in #2.