Awkwardly enough, this happened to me a few days ago, i guessed it was that id corrupted some windows files.
So i put in my Ubuntu Live Cd then moved all my files over onto my laptop. Then used my windows cd to re install it all.
But if you dont have a windows cd then you may perhaps be stuffed?
id say go to the ubuntu website and download the live cd and burn it to a cd. Then put that in your cd drive and if your computer can boot up into ubuntu back up all your data then steal a windows install cd from somewhere and re-install windows