From time to time, I like to take my PC downstairs to play some movies on the plasma.

Its a 1080i one, and plugging in via HMDI (HDMI output on card, not DVI converter).
Windows automatically selects 1280x768. If I try to select 1920x1080, it doesn't change. If I force it in the ATI drivers, it changes, but doesn't fill the screen correctly, I.E. it has black boarders all round, and doesn't look to scale.
I try to underscan it, but when the bottom and top reaches the edge of the screen, theres still slight boarders on the left and right, and doesn't look like its at 'native' res.

The TV is a Panasonic Viera, and it does do 1080i. The PS3 picks it up as that, and fills the screen properly. Also using the same HDMI cable as the PS3

I've tried setting the res under the HDMI settings of the ATI Advanced Control Panel, also trying different refresh rates, but all seems the same.

Anybody got any ideas?