Premiere Pro crashing when switching Export to H264

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Hi,

I hope someone knows what's happening. I'm trying to export a short, simple top and tail project with a logo overlaid. Every time I change the Export format to H.264 from the defailt AVI it crashes Premiere Pro - no error message just 'not responding' and I have to close the programme.

I've made sure I have the most up to date version and have cleared the media cache.

Thanks for any help.