I had the exact same issue as Atreyu and found that all the comments were very successful in resolving the issue.
I now find however that when i try to login to my default web site that i am asked for a username and password. I'm assuming that this is due to changing the IUSR_<SERVERNAME> account password, but i have updated it everywhere that i can think of. It is possible to enter the site by putting in my username password, but this isn't very handy?
Can anyone suggest where else I might need to update the password? So far I've updated it in :
Active Directory Computers and Users
Exchange (Virtual Directory)
Exchweb (Virtual Directory)
Default Web Site
Websites (Folder)
and made sure that the NT permissions are set for C:\inetpub\wwwroot
Any help greatly appreciated.