Hi Everyone,
I just wanted to share my experience with this issue. I am also using an ATI Radeon HD 5450 (although I swear it said something else before doing what I did) on Windows 7 Home Premium. I am not sure exactly how I got it working because I did a few things at the same time but I will tell you what I did.
First I went into device manager and tried to update my video card drivers. It said it was up to date. So I uninstalled the driver, rebooted, and then re-installed, rebooted. (this is where i think the video card name changed, but I can be wrong, its early morning and not enough coffee)
I then opened XBMC and I still had the problem with the black screen on full window mode.
Opened settings> video> playback and changed the following settings:
Render method : Auto Detect
Adjust Display Refresh Rate to Match Movie: Always
Shut down XBMC. Restarted it, and Voila! It worked after that.
Just to mention some other settings (which I havent tested yet if they play into my situation) but I have Allow Hardware Accelleration set to Off, and now since the fix, I can play in windowed mode or full mode (resolution set to 1600 x 900)