Connect PC via HDMI to Tosiba LCD

Hello folks,

I just got a new system:
WinXP Pro fully updated,
AMD Phenomen X2
AS Rock A785GHX wit integrated ATI HD4200 (including HDMI port) and sound

Use: Home entertainment PC

My problem occurs when connecting my Toshiba Regza 42ZV555D via HDMI to my PC: (HDMI-HDMI with a HDMI 1.4 cable purchased on amazon)
It takes quite a while until the TV recognizes the picture but it works for a resolution of 1280x1024. The picture is centered on the TV and there are black borders of appr. 15 cm (7 inch) on the left and right and 3 cm (1,5 inch) above and under the desktop.
If I select a higer resolution, the TV turns blue. This means that it cannot find any signal.

Now I'm wondering how I can get a nice picture on the whole TV at 1920x1080.

What I have done so far:
1. Deactivated the ATI Hotkey Poller service via run>msconfig>services>"uncheck"
2. Changed AV-connection in my TV's settings to PC
3. fully updated the system
4. Tried de/activating the GPU scalling in ATI CCC
5. reinstal WinXP ...

I would be happy about any help you could give me.

Thanks