You just bought a Seagate FreeAgent Theater+ Media Player. Specifications say it will play FLV and AVI but yours did not play those formats. You used a software program WinAVI Video Converter 10.5 to convert to WMV, but a machine did not also play these WMV videos.
However, it did play WMV files you downloaded directly from YouTube. What should you do to make it play FLV, AVI and converted WMV? I would install a FreeAgent Theater+ Firmware Update and FreeAgent Theater Sync software. If that doesn't resolve your issue I can try to find some conversion software that will convert playable videos as other people have this issue as well.
You install that firmware software on to your media player. Sync software is installed on your Vista computer. Both of those links have installation instructions.
You will need a USB memory stick to perform firmware upgrade. If you don't have one then you won't be able to upgrade device firmware. You can install Theater Sync software on Vista first and see if that helps first.
That firmware update is more complicated to perform. You can download this free video converter software.