Nick,
IUSR_computername account is added when IIS is setup.
Anyway then go to control panel, Administrative tools, IIS.
Expand default website and check its properties.
"Default web site Properties" - "Directory Security" tab, "Anonymous access and authentiocation protocol", there click edit. Check if Anonymous access is marked and the user name (which should be IUSR_computername). That user name is the one you'd use.
Cetin