If the source file can be played by WMP, maybe there is something wrong with the codecs on your PC. You can download a K-Lite package from
http://www.free-codecs.com/download/K_Lite_Codec_Pack.htm
Also, if you convert AVI files, make sure your DIVX codec is the latest version.
BTW,you use which version,
winavi.com/rmvb-converter.htm" rel="nofollow" target="_new">v7.6, ?