Recently I acquired another capture card - the Sweetspot (a.k.a. PDI Deluxe) - and I had A LOT of problems to make it work properly...

At the end, I used DScaler; after I was able to watch the image (tweaking with all the video settings) I closed it, opened VDub and finally it worked!

Try to do this test, and if it doesn't work I must add that I tried VDub with Windows 7 too (32 bit) and it didn't work at all, so maybe it's just a compatibility issue... I stick with ye olde Windows XP as it works with every software I need for restoration purposes.