I've had this problem for a while but since I had two computers I never to fix it badly enough to warrant a re-installation of Windows 2000, so every few months I kept trying to fix it.

- Problem, Connecting USB Digital Camera (Canon Powershot A40 or USB camera or USB Scanner) turned camera off when camera connection started. Camera turns off when connected. Preview Window turns off but light stays on. Camera was in correct Playback mode too of course.

In Control Panel under Imaging Devices I had this

As soon as I open this window the camera enters shutdown (sort of) and Status goes to unavailable. On Port: \\.\Usbscan0 (would also be \\.\Usbscan1 \\.\Usbscan2 \\.\Usbscan3 on other computers with more USB hubs)

Solution,

Go to Start - Settings - Control Panel - Administrative tools - Services
Look for "Still Image Service"
Look at the "Status" field for that service, if it is not "Started", right click and select start.
Set "Startup Type" to "Automatic" as well.

That is all. This can happen for USB Scanners too. It will happen if you play with your Services and accidentally disable Image Service thinking that it is some sort of Malware.