windows 7 pro black screen

Ok this problem started last week, my computer when I turn it on will go to the password part then after that will show a black screen with only my cursur showing. This is not a new installation. have had the computer almost 2 monts. finally it started ok after 5 attempts. Now it is doing this again. I have completely removed the computer from power for 10 minutes, and then tryed again but still comes up black screen. At one time it offered me the option for safe mode and I brought it up in safe mode, checked all my programs and they worked fine, powered down, then back up and got the black screen. any help would be great! Also this computer is hooked to a network, the whole network shuts down when this computer stops working. I am on another computer now that is directly hooked to the internet away from the networking.

Ok I think the problem might be my networking card. How do I find out if mine is working ok