However, since installing it, if I leave my PC alone for about 10 minutes, it takes me back to the log-in screen
This is Windows default behaviour (at least for Media Center Edition). Has nothing to do with ATi. My laptop does this (or rather it used to) and it has an nVidia chipset.You can disable fast user switching if you don't want it to do that.
Not sure why they couldn't do this with the XP version as well.
Since there is so little difference between the two OSes, I'm going to go out on a limb here, and suggest that DX10 is probably why.