Why do I have problems playing DVDs on my computer

There are thousands of answers to this question, but here are some basic troubleshooting steps to help you track down problems such as jerky playback, pauses, error messages, and so on.

Get updated software. Driver bugs are the biggest cause of playback problems, ranging from freezes to bogus error messages about regions. Go to the support section on the Web sites of your equipment manufacturers and make sure you have the latest drivers for your graphics adapter, audio card, and DVD decoder (if you have a hardware decoder). Also make sure you have the latest update of the player program.
Apple has released numerous updates for audio drivers and the DVD player application. Make sure you have the latest versions. Go to the downloads page and search for DVD.
If you have problems loading a DVD on a Mac, hold down the Command, Option, and I keys when inserting the disk. (This mounts the disc using ISO 9660 instead of UDF.)
Make sure DMA or SDT is turned on. In Windows, go into the System Properties Device Manager, choose CD-ROM, open the CD/DVD driver properties, choose the Settings tab, and make sure the DMA box (for IDE drives) or the Sync Data Transfer box (for SCSI drives) is checked. Download CD Speed to check the performance of your drive (if it's below 1x, you have problems).
Caution: You may run into problems turning DMA on, especially with an AMD K6 CPU or VIA chipset. Check for a BIOS upgrade, a drive controller upgrade, a bus mastering driver upgrade, and a CD/DVD-ROM driver upgrade from your system manufacturer before turning DMA on. If the drive disappears, reboot in safe mode, uncheck DMA, and reboot again. You may have to tell Windows to restore the registry settings from its last registry backup.
If you get an error about unavailable overlay surface, reduce the display resolution or number of colors (right-click desktop, choose Settings tab).
Try turning off programs that are running in the background. (In Windows, close or exit applets in the system tray -- the icons in the lower right corner. In Mac OS, turn off AppleTalk, file sharing, and virtual memory.)
Allocate more memory to the Apple DVD Player.
If you are using a SCSI DVD-ROM drive, make sure that it's the first or last device in the SCSI chain. If it's the last device, make sure it's terminated.
Reinstall the Windows bus mastering drivers. (Delete them from the device manager and let Windows ask for original disc.)
Bad video when connecting to a TV could be from too long a cable or from interference or a ground loop. See 3.2.2.

More information on specific graphics cards and driver updates: