Converting to WMVHD and showing on my TV Black Bars on Sides

[elikd]

HI guys,

I convert a 720P video to WMVHD 720P. It works fine. For some reason on my widescreen HDTV it shows with black bars on either side. Now the TV is native 1080P but it should automatically display whatever resolution it gets in the full size. The same video converted with Vegas Video to the same format and resolution does not do this (show the black bars on either side). The only think I can imagine is some type of flag or something that is making my tv process it differently. Any ideas????