Ray Adams, the creator of
ATi Tray Tools is not supported by ATi/AMD (I.E ATi/AMD not giving him free hardware for his tools not even temporarily), sadly, so he needs the hardware to make the tool compatible with it, and he also needs the OS to work on and make the tool compatible with it as well. As far as I know he does not own a Radeon HD2900XT, so R600 support on ATT won't happen for a while. And I don't think he has any copies of Vista yet, but I might be wrong (or else I'm sure ATT would fully support Vista already).
With that said I noticed that you got a Radeon X1900XT in your rig, which is supported by ATT but only on Windows XP, I think. I suggest you go take a look at the discussion forum where Ray Adams himself hangs around most and perhaps ask him directly with a private message (of course you'd need to sign up to that forum if you're not already), here's the link:
http://forums.guru3d.com/forumdisplay.php?f=33