If lg99's suggestion does not help, you may need to go to one of the free Malware removal sites listed below. They are able to see if your system has been attacked and that is what is hiding your Windows installation, or if it is missing, why.
Choose one of the sites and follow the directions exactly. Do not try anything else on your own, as it will make any recovery more difficult. Remember the sites are all volunteer run, so it may take a little while to get the solution.
http://www.bleepingcomputer.com/
http://www.geekstogo.com/forum/
http://www.cybertechhelp.com/forums/
http://forums.whatthetech.com/