I have still not resolved this problem but I have now found that exactly the same problem occurs with photostage.
Two different programs, same problem.
Runs fine on my i5, but not on my i7.
I'm wondering if it's a machine setup problem. I've tried reinstalling video codecs but that didn't help - any other ideas on what I can try turning on or off / reinstalling / updating to try and get these two programs working?