I don't have enough information.
Did you purchase a skin changing program and install it? What exactly did you do?
Is is possible that you accidentally inversed (solarized) the screen using the hot keys provided in Universal Access? Finder--> System Preferences->System-> Universal Access->Seeing Tab>Display Black on White?
What is in your start up list? Finder--> System Preferences-> System> Accounts>Login Items
Post a screen shot Finder -->Applications->Utilities>Grab or (Shift+command+A or shift+command+W).