Hi,
1. What do you mean by "compatible DVD decoder install on your computer.FREE"?
2. Are you getting any error code or error message?
3. Since when are you facing this issue?
4. Have you made any changes to the computer recently?
5. What happens when you try to play the DVD?
6. Which program are you using to play the DVD?
7. Are you getting any error message while playing the DVD?
I would suggest you to try the steps provided in the following Microsoft article and check if it helps.
How to troubleshoot common problems that occur when a Windows XP-based computer cannot read a CD or a DVD: http://support.microsoft.com/kb/321641
Also try playing the DVD in a different computer and check if it helps.
Hope the information is helpful.