IIS 5.0 error message: Service Unavailable

Solution:

1. Open the IIS Manager, right click Web Sites on the left panel and select properties.
2. Click on the Service Tab.
3. Check "Run WWW Service in IIS 5.0 Isolation Mode".
4. If asked to Restart of IIS then click yes else click Start --> Run --> type 'iisreset' and press Enter.

Try opening the website again and you will be perfectly fine.