Some things to check or at least think about:
– make sure the user accounts you are using for setup are in the correct user groups
– if you are doing a multi computer install you are using domain accounts like domain\\name
– if you are re-running the installation make sure all the databases and user accounts that were automatically created have been removed
– you have all required components installed
– you have not installed any beta software (like Biztalk 2006, VS 2005, .net 2.0) on the computer prior to installing Biztalk
It almost sounds like something might not have gotten installed correctly? If nothing else works, you might want to uninstall and reinstall Biztalk 2004.
Hope this helps.