Who can help me with a codec problem on the program Riva Encoder 2.0

Okay what I do is take episodes off youtube and make amvs with them in WMM. So I save them as a .flv then I put it in Riva FLV encoder and under Destination Video file i change .flv to .wmv and then it says,
The encoding of your video has failed.
This can be caused by a not supported combination of parameters or by a not supported Video codec.

I got this message on an older computer but then I downloaded a codec and it worked, I just dont know what codec that was.