First avast doesn't block, it isn't a firewall, it merely scans and alerts to infection (at which point it would stop you doing anything with the file). So can you be more specific on exactly what is happening or not happing, e.g. what you did and what happens.
I feel you may be getting the easy answer from proshow when they say 'your security program is stopping you uploading,' that is just to much of a general answer. What they should be asking is what security applications do you have and then say what security applications might be interacting with proshow and more importantly why it might be happening and what to do about it.
The 'sometimes it happens sometimes it doesn't is a really great example of technical support 'not.'