Format of video downloaded from Keep-Tube/KeepVid/Savevid?

Does anyone know how to convert the HD .mp4/.flv video downloaded from the sites mentioned above? I have tried to use several kind of converter but it seems they cant detect if the file is even exist. I want to make the converted output to be able to integrate with VirtualDub/Mod because I want to sub it. I used Total Video Converter & SUPER since both of it can make the .avi to be used for encoding. But they cant seem to recognise my video files. Although another converter is able to detect the video, iwisoft, but the output is not suitable for me to use for Vdub/mod, in other way, for encoding. So, I really need a help on the best & suitable converter for me to us. & please dont try to spam just because you want to increase your point with a crap answer. TQ.