How to config application pool in IIS 7.5 automatically re-start when it was stopped?

You will need to change the startMode for the application pool your website is running on from onDemand to AlwaysRunning. By default IIS sets all application pools to onDemand.

1. In IIS Manager, click computer name in the Connection pane.
2. Switch to Features View if the view is not active.
3. Double-click Configuration Editor in the Management section of the Features View.
4. Click the down-arrow for the Section field, expand system.applicationhost, and then click application pools.
5. Click (Collection) and then click ellipses (…) next to the field that shows the count.
6. In the Collection Editor, select the application pool for which you want to configure the startMode attribute.
7. In the Properties window at the bottom, set the value of startMode attribute to AlwaysRunning.

Source: http://msdn.microsoft.com/en-us/library/ee677285%28v=azure.10%29.aspx

In IIS 8 and above, the start mode setting is now directly available from the Application Pool’s Advanced Settings.