No sound device on windows 7

Hello, i own a cs l n1008 slim ,after installing windows 7 from x p ,my sound is gone ,i see a red cross by the corner ,under sound games and controller .only says high definition audio.i tried to download realtek like sme1 told me .but nothing has changed .Please help me um stressed the cross says no audio output device is installed,my configuration is Intel atom N270 1.6GHZ.............