Try opening bridge only. Close everything else. If the previews work then you are running out of system resources. This is not an uncommon problem with only 4GB ram. Ram is cheap and easy to install in an iMac...
Thanks for the quick reply Rick. Yes, 4Gb is not a lot, but when I bought the machine I was not doing any video and it sure seemed like plenty in 2008. I probably should upgrade...
I did close down everything else, but the problem still exists. I can navigate to a thumbnail, but once I click on it and try to preview, the program freezes and I have to force quit.
Make sure everything is up to date. Check your activity monitor for apps that are tying up memory and CPU power. Try downloading the free app Memory Diag to free up memory.