Service Unavailable message in IIS when rendering MOSS sites

This is something that I found while troubleshooting W3SVC errors. I was getting a service unavailable message for any site that I hit using in our SharePoint environment. Thought initially it is an application pool crash and set up the dcdiag tool to capture dumps, but later learnt that "service unavailable" is not picked up by this since dumps are created only if the website takes time to render and service unavailable is almost instantaneous. Tried by just reentering the service account password in all the application pools and an IISRESET later the site came up. Strange but this did work.