Trouble playing DVD movies

I just inherited an HP notebook, Pavilion ZE5375us, from my mom (who had to upgrade, even though this notebook is just a few years old) as I needed one for school. She mentioned that she never could get it to play DVDs, but the CD-Rom part worked fine.
It does have a DVD reader (QSI SBW-241) in it. I spent a better part of today downloading a file for it. I was able to get a DVD to play,but it was a data DVD, not a movie. I have InterVideo WinDVD 4 installed. The microsoft website is telling me that the WinDVD 4 decoder is not longer valid. So, I downloaded the free trial of WinDVD 7 (to make sure that is what is going on) and downloaded Media Player 10 to go with it. So far, I have not received an authorization code from InterVideo… I just want to play a movie on my computer. I don’t even know if my DVD drive is capable of DVD-R… is there any free decoders out there for Windows XP that I could download to just play a DVD movie? Any help would be greatly appreciated.

Update Frimware.
If that doesn’t work. Try VLC Media Player to read the dvd.

Media Player Classic with ac3filter and ffdshow should do it. Google for those three.