This is an Win XP SP2 that was installed a LaserJet 3030 in 2005; it worked fine, but at some point Twain stopped working. Scanning using Twain gave "There is no Twain-compliant device installed" error, but scanning through Windows Scanning Wizard (without Twain) works.
We completely uninstalled the LaserJet, and even deleted all Twain folders and files from the Windows folder (those get recreated) - but reinstalling didn't fix it.
Then, after another cleanup, we installed a Canon 9950F scanner. This one shows no source in the "Select Source" dialog, again like there is no Twain device.
How can we repair the broken TWAIN ?