The articles/posts I've seen lean towards 2 things: 1. The laser is dodgy/losing power, reading CDs uses a different wavelength to reading DVDs. Normally I'd say that'd be if its a few years old though. 2. The lens is dirty. Cleaning things from Walmart for $10 or so seemed a popular answer.
Depending on how geeky you want to be, you could rule out a hardware problem (maybe) by creating a boot CD that has a DVD player on it. If the boot CD can use your DVD drive, then you know that the issue is a software one and is either your OS or your Player software.
You could try this one Geexbox (geexbox.org) or look for a different one.
My computer is suddenly displaying an "antivirus" HTML document over the top of my desktop (it's blocking the wallpaper I made, but allowing the icons to show through). Properties reveals that this HTML thing is: C:\WINDOWS\desktop.