My computer is saying it cannot find a sound device? My speakers are plugged in, and in the right places.Help!?

I recently DLed Windows XP home edition from a friend's CD and now my sound isn't working. My speakers are in the right places, and the blue light comes on, I know they aren't broken because I can hear the little noise when I plug them back in. And when I try to play a song on WMP, it says "Windows Media Player cannot play the file because there is a problem with your sound device. There may not be a sound device installed on your computer, it may be in use by another program, or it may not be functioning properly." Does anyone know how I can configure my sound settings or search for my speakers on the computer??????