Well, I've done some radical stuff. Uninstalled 2013, unplugged Impulse and removed all mention of the Impulse from the registry. Then I installed 2010.5 and plugged in the old keyboard (Roland PC-200 MKII). This is not a USB keyboard, it plugs a DIN directly in the V-Studio 100. This was the setup that used to work fine. But it doesn't now. Exact same problem with input. Still, seems to me I did in a more radical form what Noel96 suggested.

Then I uninstalled 2010.5 (removed /bb at the end manually) and reinstalled 2013. And we're back to square one.

Also, I disabled realtime scanning in McAfee, but no results. I still have to figure out how to add bbw.exe to a list of trusted programs in McAfee; usually you only get that option when a file is suspect.

But, at least I'm now pretty confident it's not an Impulse-issue, since I can't get input from 3 other instruments. The weird thing is of course, that it all started with installing 2013. And whatever it was that did it, it can't be rolled back.