There is no password, and the account is disabled by default (which is why you're having probs)
You need to go into Computer Management --> Local Usesr and Groups --> Users and double click the Administrator and enable it and set a password.
If you have Vista Home ____ then you'll need to do this differently
Click Start - type cmd and press Ctrl+Shift+Enter
type:
net user Administrator /active:yes
log off - and logon as the REAL Administrator and change its password via Control panel
*Note - Computer Management is accessed by right-clicking My Computer and choosing Manage.