Does vista come with a built in dvd player?

Yes it’s called windows media player.

my friend put in a dvd and he double clicked it and windows media player said it doesn’t have the right dvd decoder. do you know what that’s all about?

Depending on the version of vista he is running it may not include the MPEG-2 Decoder. See this page for more info.

My advice to him would be to install VLC Media Player which should solve his problems if I am not mistaken.

Corel WinDVD 9 is a great program as well.

If he has vista home premium or ultimate it should play in windows media centre. If he doesn’t have the correct codecs download and install a demo of PowerDVD. Then you can uninstall it and the codecs will remain so he can then use windows media player. I recommend keeping PowerDVD though – at least until the trial ends as it’s great for playing dvds. Otherwise VLC player as mentioned by Battlemarz is excellent as it plays all formats, including divx, mkv(usually 1080p stuff is in this format) and it even plays Mov/mp4 format better than Quicktime.