The two most common Windoze DVD playing softwares are PowerDVD and WinDVD. They both work fine. If you buy a computer with a DVD drive it should come with one of them. Stand-alone DVD drives for desktop computers usually come with one of those packages bundled with the drive.
There are a couple web sites that you may find interesting with useful info for DVD playing. They are:
www.doom9.net
www.afterdawn.com
www.cdfreaks.com
www.zeropaid.com
There's also a site called
www.rpc1.com or some such that you may find interesting. Have fun. Beware the greedy MPAA.