If you can see it while Windows is loading, the AMD video drivers haven't loaded at that point. It could be a bad cable. It could be bad drivers. It could be a bad port on the TV or the video card. It could be an adapter issue (Is it DVI or HDMI to VGA?).
Any or as many answers you can provide to the following questions will help troubleshoot this issue.
Does it give you the "unsupported signal" message at about the time it should be asking for your login or when the desktop would be displayed (if you don't have multiple logins on that PC)?
Did you just start using the TV as your monitor? Or has it worked in the past? When did you last update your video drivers? Did you get them from AMD, your video card manufacturer, or Windows update? Do you have a PC monitor you can hook it to with a VGA connection to see if it's a consistent issue across displays? Does the TV work with other devices running through this VGA connection?