Which version of WMP are you using? Is the computer still working fine except the WMP issue? If so, try using a different media player such as the free one at the link below:

http://www.videolan.org/vlc/

Please note that a DVD decoder is not installed by default on all versions of WMP.. Have you checked to make sure you've got one?

The video adapter warning is asking you to troubleshoot the video driver, etc.. Try updating the video driver to the newest for you device.

In addition, the links below should help:

Troubleshooting DVD playback in Windows Media Player for Windows XP

How to troubleshoot the video adapter driver in Safe mode in Windows XP

Hope this helps.

Grif