Your display drivers are probably the stock Win7 drivers, which are not suitable. Figure out what graphic card you have installed & go to the card manufacturer's website to download the latest recommended drivers.
Thanks, but I´ve found a problem with my Decklink Extreme PCI card. The Premiere preview problem happens only when I´m using any Black Magic presets. Is Decklink Extreme PCI/windows 7/Adobe CS5 a bad combination? I´m not able to get any picture on my monitor from the decklink what so ever (composite/component), but there´s a signal there (black video). Worked fine in XP (with CS3), kind of worked in win7 with my old configuration (4GB RAM, single core AMD-processor), and absolutely no response now.
There was a bug in CS4 whereby preview on an external monitor would freeze the playback on the internal monitor. Essentially you couldn't have both. This bug was introduced with CS4.
It sounds like it still may not be fixed.