I spent the past day trying to figure out why I couldn't log in as myself, Sharepoint would always just say I was logged in as System Account. After a ton of research and changing things I ran into this:
To fix this, you must run the following commands: stsadm -o updatefarmcredentials -identitytype NetworkService
followed by: iisreset
http://social.technet.microsoft.com/Forums/en-US/sharepointadmin/thread/8d8d9b30-0995-47ba-96e8-7b52872664f3
That solved my problem. I'm using NTLM authentication and I set up a new Active Directory user called sharepointuser to be the identity for the app pool (changing the app pool from myself to that user probably helped, but that isn't what fixed it ultimately). I'm pretty sure if I had set up sharepointuser before I installed MOSS 2007 on the Windows 2003 server and specified that user during the installation, I wouldn't have had the auth problems to begin with.