ExporterAVI. Deleting this fixes the problem. Prem would hang when loading this component so I moved it and hey presto, things works fine, The same goes for AE and Encoder.
Can I make a suggestion that AE should list each component as it is loading them rather than just0 'Initializing Mediacore'? If I hadn't have tried booting Prem which does list each component, then I would still be wondering why it wasn't loading.
I haven't used an AVI since around 2002 so it doesn't bother me not having it but I am rather puzzled at why this was causing the problem, especially after doing a clean install.