Best thing to do would be to get to the Dell website and locate the specs for your PC.
Look here for more info:
In the old days every Dell PC had a unique number that you could type in at the website and you'd get your personal support page with drivers etc. I'm not sure if it still works that way, but in my experience contacting Dell (over the years I've bought about 6 PCs through Dell) is a very easy thing to do and they're very helpful. Look at the support page for details.
It might also have something to do with the OpenAL drivers, read through this and see if that helps:
Although I suppose that your problem isn't really connected to that, given the error message you seem to get.
Finally, nothings just simply stops working, there's always a reason. Think back: did you install anything before DoP stopped working, did you change anything? Even the most insignificant thing can mess things up.