Thanks, Ann and Stan for your help.
I ended up resolving this by exporting new AVI files from using a different computer and then i replaced the exising assets in my Encore project. It seems that that files I had originally created from my Windows 8 machine were causing the problem. Using the new files created with my Windows 7 machine worked. I know that sounds strange but that's the conclusion I came to. I'll experiement more with Windows 8 later. If i come to a different conclusion, i'll post it here.