Can someone please help!!

I have d/l a couple of .avi's. If I run them through Nero to encode is doesn't seem to recognise them. If I run them through TMPGEnc it encodes them to mpeg. I use Nero to burn them, but when I play it on my DVD I get sound but no picutre. The whole clip plays perfectly on my pc prior to this... Can someone please tell me what I could be doing wrong??

Many thanks

Norfy