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.
I'll save all of you Dell owners the hassle and post the link here. Spread the word to your friends about this site. Click Here (dellbatteryprogram.com) to go to the recall site. Follow the directions and you should be fine.