Information
IIS introduced a new security feature called Application Pool Identities that allows Application Pools to be run under unique accounts without the need to create and manage local or domain accounts. It is recommended that all Sites run under unique, dedicated Application Pools.
Rationale:
By setting sites to run under unique Application Pools, resource-intensive applications can be assigned to their own application pools which could improve server and application performance. In addition, it can help maintain application availability: if an application in one pool fails, applications in other pools are not affected. Last, isolating applications helps mitigate the potential risk of one application being allowed access to the resources of another application. It is also recommended to stop any application pool that is not in use or was created by an installation such as .Net 4.0.
Solution
Open IIS Manager
Open the Sites node underneath the machine node
Select the Site to be changed
In the Actions pane, select Basic Settings
Click the Select... box next to the Application Pool text box
Select the desired Application Pool
Once selected, click OK