On Windows XP computer, from time to time when opening up Internet Explorer ( 7 or 8 ), the top part of the screen starting from the tool bar line is all dark. This is a Windows problem, not an IE problem.
To fix it, the simplest way is to hold down the left Shift, left ALT and hit the print screen key, this will pop up a windows that says do you want high contrast mode, click OK.
Now the screen looks even worse than before
Hit the left Shift, left ALT and hit the print screen key again, now turn off the high contrast mode, and your original screen comes back without the blacked out Toolbar.
More detail here.
http://www.hardwaresecrets.com/article/Fixing-the-IE8-Black-Bar-Bug/816