A pc tv card typically only supports regular NTSC signals.
Or, if it's a nice one, PAL and NTSC signals.
I imagine dpaint is using a different screenmode, or perhaps it is running in PAL when you have only an NTSC tv card.
You might want to start your Amiga in NTSC mode, (Hold down both mouse buttons when starting, and hit the space bar to tolgle between modes, and wait to see if a signal pops up on via the wintv card.) I'm not sure if this is applicable to all models, ie A500, A600 etc.. but I think so. Also, try using the composite video signal from the amiga instead of the rf modulated one, or vice versa..
