iPhoto 08 video playback has stopped

Any ideas how I might persuade iPhoto 08 to start playing back videos again when double clicking on the video? It used to fire up Quicktime and play MPEGs and AVIs quite happily but now it does absolutely nothing at all (and hasn't for some months).

No new software loaded at the time it stopped as far as I can remember.

Thanks in advance for any help.

- iPhoto 08
- OSX 10.6.8
- iMac 2.4GHz, 4Gb RAM