DVDs won't play on my PC. How do I fix it?

Hi. I have a Dell PC, and Windows VIsta is installed. I can play CDs on the drive, but not DVDs. They did play, just don't now. If I put one in, then try to open it through computer/C/dvddrive, then you can't even see which disc is in, and the tray opens. I have re-booted, looked for drives to install, tried the MS fix from their site. I have tried this on both Media Player and VLC, but the key issue is that the disc isn't even recognised by Windows, before we even get to the player. Anything else to try? Thanks