Can't watch movies after Vista --> XP

Hello,

I can't watch movies any longer after tech support downgraded my HP Pavilion laptop from Vista to XP. It didn't help that he didn't make a backup of the device drivers. I've managed to find all drivers, the machine works excellently, but whenever I load a DVD and open it with Windows Media Player 11 (WMP) it doesn't play. I just get directory of the movie chapters.

I ran a Microsoft Windows XP Video Decoder Checkup utility and learned I didn't have a valid decoder. So I purchased one from Corel that came with WinDVD (2010). The utility indicates I now have a valid decoder.

However, when I try to watch a DVD with either WMP or WinDVD they both crash with the same error: "Windows Media Player has encountered a problem and needs to close. We are sorry for the inconvenience." and the information I can find is "Exception Information Code 0xc00000005"

I read a post that said indicated it may be an issue with my audio or video codecs. OK. I made a Restore point, deleted all of the Video and Audio codecs (except for one Microsoft Audio codec I was not allowed to delete), downloaded K-Lite codec pack, and guess what? Nothing's changed?

I've downloaded Housecall and Ad-Aware to see if there's some malware, but Housecall is clean and I suspect Ad-Aware will be, too.

I just found this disheartening link: http://h10025.www1.hp.com/ewfrf/wc/document?lc=en&dlc=en&cc=ca&docname=c01092639&printable=yes&product=3227473

I have an HP Pavilion dv2500 (also says dv2715nr further down)
Windows XP

I do not have any P2P sofware, Firefox, Mozilla...just IE7, Office on this machine.

Not sure what else to list here.

Any help on this matter is appreciated as I'd like to give this laptop to my son and it seems kind of crumby that it will not play a movie.

Thanks,

Jose.