What model of television is it?
Mode Not Supported means that the resolution that the television is receiving is an unsupported resolution. If you change your resolution, you may be able to get a better picture, but I think your best bet is to use DVI if it's available.
I have an issue with connecting up my computer, equipped with new ASUS TV out, HDTV out compatible graphics card to my Samsung rear projection 'HD Ready' TV which in theory supports 1080i input throught component video.
The TV, HDTV out function from the graphics card is by default from the 9-pin S-Video port via a component video adapter cable, fairly standard practice as I understand.
(I did wonder if outputting from the DVI port would be any better but don't want to waste the money on a cable if I don't need it).
The next highest resolution format that the TV supports is 576p.
When outputting 576p from the computer everything works well, besides the fact that objects are displayed quite large due to the low resolution but this is fine, I can quite happily live with that. The thing that really bothers me is watching videos, which was the main reason for connecting the computer to the TV in the first place. I'm sure you will all understand that 576p on a 42" screen doesn't look that great and so if I can get the 1080i to work that would be awesome.
The card is definitely supposed to be capable of outputting HDTV but when I change the signal output to 1080i then I get the NOT SUPPORTED MODE message on the TV screen (so I'm guessing the issue is the TV rather than the computer/graphics card).
I havn't yet tried hooking the computer up to a different TV, or a different 1080i source to the TV but to my mind there is no reason why what I'm trying to do shouldn't be working...
If anyone could help me out it would be greatly appreciated, thanks!