Suggestion: Tell us what DVD drive you are using (Manuf and full Model #), also what software player have you tried, which one came with the drive. Slim chance that the software that the came with the drive doesn't work properly if your system is set up and hardware/software is installed properly.
Have you exhausted all the manufacturers resources (hardware and software), is DMA enabled, is the DVD drive the master, etc...? Is there any updates available to the DVD drive (I realize its new - but you don't know until you look)?
What video card? Does it meet the minimum specs for playing DVD's (as well as your entire system)? Check out the video card website - ATI has some very basic yet helpful troubleshooting checks.
Before this DVD drive installed did you have a DVD rom installed? Did it play DVD's properly? Are you trying to play commercial DVD's?
Hope this doesn't come across as *itchy - not intended to be. You are your best advocate and it will be you who ultimately solves the problem.
r