Login failed for user 'IIS APPPOOL\ASP.NET v4.0'.

Description:
An unhandled exception occurred during the execution of the current web
request. Please review the stack trace for more information about the error and
where it originated in the code.

Exception Details:
System.Data.SqlClient.SqlException: Login failed for user 'IIS
APPPOOL\ASP.NET v4.0'.

Source Error:

Line 44:         </tr>
Line 45: 
Line 46:     <% foreach (var item in Model) { %>
Line 47:     
Line 48:         <tr>

Solution:

Here's a step-by-step guide to determining your websites Application Pool, then changing its Process Model Idenitty in IIS7:

1. Open Internet Information Services (IIS) Manger.
2. In the Connections sidebar, drill down into Default Web Site and click on your website.
3. Now in the Actions sidebar (on right side), click on Advance Settings... In the popup box, under General you will see your Application Pool listed for your website (in my case the app pool is: ASP.NET V4.0).
4. Click Cancel...  If you choose, you can change the Application Pool here, but for the sake of this example we just wanted to find out what the website's App Pool was.

Now that we know the Application Pool that is configured for are website, we now want to change the app pool's (Process Model) Identity to 'NetworkService', here's how:

1. Open Internet Information Services (IIS) Manger.
2. In the Connections sidebar, click on Application Pools.
3. Now right-click on theApplication Pool that your website is using (in this case my site is using the ASP.NET v4.0 application pool), and select Advanced Settings... from the menu.
4. In the Advanced Settings pop-up box, locate the Process Model -> Identity section and click on the Application Pool Identity. 
5. In the Application Pool Identity pop-up box, change the Built-in account to NetworkService (or if you want LocalSystem), then click OK, and click OK again to save your Advanced Settings changes.

After changing the Application Pool's Identity to NetworkService (or LocalSystem) you should now be able to get your ASP.NET site to run successfully on a IIS7 web server: