Thanks for the model #.
Second, you might be screwed. Compaq DOES NOT provide drivers for your DVD on their site, after looking at the drivers which they provide...though they do provide display drivers apparantly (though I thought you said you put the computer together...if you put a new card in, their drivers probably aren't the drivers that are exactly compatible with your new card I'd imagine, but probably might work as generic drivers for your display and perhaps your card as well as their other drivers).
Unfortunately they don't specify what DVD player they have installed. You might try to install a Codec (intermedia comes on some DVD's for free, and works with WinXP, unfortunately I can't remember which DVD's it was packaged in).
However, if it is the driver...you might call Compaq and find out what exactly the DVD player you have and what brand it is. Otherwise, there is a small chance that somewhere on the drive, there is a small tag which tells you either the brand, or has a few numbers which you can look up on the internet to tell you the brand of your DVD player. If that was stripped off, than it gets WAAAAY more technical on determining the brand.
However, one way, and BE CAREFUL is to go to your device manager via your system from your control panel and see what it has listed. If it is a driver conflict than the appropriate DVD player will be listed and you'll know that you stand a chance that it could be a codec or driver conflict, in addition you'll probably know what DVD player type you have. This WILL NOT WORK however if you've already installed other drivers over your own drivers, or in otherwords, installed new drivers already, without knowing the brand of your DVD player, in which case your screwed.
Now, if it says generic drivers, there are other ways, but I don't want to be typing all night on how you can determine what type of DVD player you have, when it could be easier just to call compaq, or better yet, buy a new DVD drive, install it, and use the drivers that come with it, along with the DVD codec program.
Oh, and good luck.