I removed, then reinstalled SharePoint 2010. Attempting to recover an existing web application, I reconfigured the web application in the new instance, and pointed the existing db to that web application. In IIS, I created a new app pool, together with a new account to run it. In Sql Server, I created a security account for that new app pool account, and assigned it to the relevant database.
I start the app pool in IIS, and it runs. I restart the web application, and that runs, too. I then attempt to access the web app, and the app pool stops. The browser reads "HTTP Error 503. The service is unavailable."