Never had that with the DN Converter. But I experienced trouble of this king with other programs, when running out of "resources". Try to reboot (switch the machine of/on, to reinitialize the hardware completely).
I wish I could say that fixed it, but it didn't. It's STILL doing the same thing.
Sorry, but I have no imagination where to look next! Did the system by chance upgrade the display device driver? Did you try an older version of DNG convert?
I looked in my Roaming directory, but could not find anything that coul be deleted to figure out what the problem might be... :-(
I have EXACTLY the same issue with DNG 8.3 and WinVista.
I have downloaded and re-installed 8 times with variations to no avail.
Can ANYONE help with this?