SharePoint: "Service Unavailable"

When you open a browser and navigate to a SharePoint site and get "Service Unavailable" this is usually due to a bad application pool account: wrong account, expired, bad password etc. I found a new cause... MOSS evaluation copy has expired! I got this error trying to run a quick demo on a VPC I had not used for a while. I then wasted a lot time exploring the accounts, changing passwords etc. Then it dawned on me that this VPC may have been built with an evaluation copy of SharePoint. Sure enough... set the date back on the PC and all works fine. Now off to use my correct VPC... For other causes just Goggle "SharePoint Service Unavailable".