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'm thinking of getting one of these when I get my new laptop. Does anyone have one or heard anything from someone who does? http://www.shopping.hp.com/webapp/shopping/store_access.do;jsessionid=EaaMC6zulhfW2lskRlpzcrQPARDVj3BGsltC1KwApmFvSUtqmPTH!