I have finished creating a complete YouTube clone in .NET. The most complex part of the whole project was to find out how to convert media into FLV after upload. This research took the better part of my project time. Nonetheless, it is complete in terms of functionality, though UI needs some major rework.
Anyhow, what you need is FFMPEG.exe. It is a command line utility that you can run as a background process, or schedule a windows service to call it. FFMPEG requires command line parameters. Here are two examples of how you would convert using FFMPEG.exe.
ffmpeg.exe -i new5.wmv -f flv -vb 20000 -s 320*240 -aspect 4:3 -r 30 -sameq Sample20.flv
ffmpeg.exe -i new5.wmv -f flv -ab 512 -ac 1 -ar 44100 Sample19.flv
To get thumbnail from video, use the following code to extract an image four seconds into the video:
ffmpeg -itsoffset -4 -i demo2.flv -vcodec png -vframes 1 -an -f rawvideo -s 320x240 test.png
Download FFMPEG.exe (google for it) and thats it. Apart from this, the rest of youtube clone functionality is not complex and straightforward.
If you need more assistance, please let me know.