IIS 7.5 runs Application pool under AppPoolIdentity.
The Windows operating system provides a feature called "Virtual Accounts" that allows IIS to create unique identities for each of its Application Pools. Click here for more information about Virtual Accounts.
Useful articles:
http://blogs.iis.net/webdevelopertips/archive/2009/10/02/tip-98-did-you-know-the-default-application-pool-identity-in-iis-7-5-windows-7-changed-from-networkservice-to-apppoolidentity.aspx
http://www.iis.net/learn/manage/configuring-security/application-pool-identities
http://technet.microsoft.com/en-us/library/dd548356.aspx
