If you've recently added a new USB scanner to your Windows2000/XP system, and have tried just about everything to get it to work (upgrading or downgrading to a different usbscan.sys, updating USB chipset drivers, etc) and you still see the dreaded 'USBSCAN0 not available' here's one last idea.

Make sure the Still Image Service is running.

Once started, everything will work.
Strange how this isn't checked for by scanner software installation packages...

Have fun!