How do I get a free dvd decoder for xp?

Yes.

Download the K-Lite package. http://www.free-codecs.com/download/K_Lite_Codec_Pack.htm

The new package contains both 32-bit and 64-bit versions so it should install just fine. I recommend you use the DEFAULT settings and use the SOFTWARE decoding (don't check H.264/MPEG2/VC1) unless your CPU is too old and causes stuttering AND you have a compatible video card.

There are a few SETTINGS you should change in the new WMPC-HC media player which should be the default such as:
1) Launch FULLSCREEN
2) Remember DVD position, and Remember FILE position (start where you left off if close and reopen)