Well before restarting I noticed javac running at 30% CPU usage in Task Manager, I killed that task process and got a 'virus detected' warning from Avast (this happens pretty much daily for over a year now, can't figure out why). Tried again but same problem with 'Open' in B4i.
Restarted PC, same problem. I did notive the the 'Open' explorer window took about 20 seconds to render and become responsive a few days ago, but it did eventually become responsive - not happening now! Should I try uninstall/re-install a particular version of .net?