The first time I installed Server 2008, I thought to myself, "I don't remember being asked to set the password." So I installed it a second time and am now certain that it must be asking for a default password.
2 Answers
The default Administrator password is blank and you're required to change it on the first logon.
I know this probably doesn't help too much but after you set a password here is some info on the password policies for 2008 server.
Like Evan said, the default password should be blank unless you accidentally put something in without realizing it.