If it's like the ones I've seen before, getting rid of the desktop problem is pretty easy. There should be a tiny line at the very top of your screen that is actually what's left of your desktop. What appears to be your new desktop is actually an installed desktop item. So you move your mouse cursor to the top of the screen where the tiny line is, right click, choose Properties, and you should get your Display Properties window. Go to the "Desktop" tab and click on the "Customize Desktop" button. Then click the "Web" tab. Delete every item in there except "My Current Home Page," which is probably unchecked anyway, since you're not likely to want to display that as your desktop.
IIRC, there should be an HTML file in your C:\Windows folder that is actually the desktop item. But this may not be the same threat.
Then you'll probably want to post a HijackThis! log in the Hijack forum here at Bleeping Computer to get cleaned up. The black screen is the easy part.
When you go to the link, make sure you read the pinned post, "How to Submit a HijackThis log." Go here.