Hi,
This article may help you.
http://learn.iis.net/page.aspx/140/understanding-built-in-user-and-group-accounts-in-iis-7/
Note that if the site is being served from a network share, the IIS_USERS does not have network rights by default as described in the article.
On a dedicated server, I typically just use NETWORK SERVICE as the identity on the application pool and then give this user the needed file system permissions.
Hope it helps,
Joe