Your homepage has been changed to something else? And your wallpaper is missing? This sounds very much like the symptoms of an infection. I suggest you follow the steps below first, to see if that helps. If no infections are found, then please come back to this thread. If there are infections, then please post in the malware removal forum.
Please go to the malware
forum and follow the instructions at the top....Especially the CLICK HERE
That will give you several steps that will help you clean up 70 percent of all problems by yourself. If at the end of the process you are still having difficulty--and you may not be-- then post a hijackthis log in THAT