Problem playing video dvd on windows xp forum

Dear Forum Members:
Attempting to play store bought DVD on Windows XP (32) using Microsoft Media Player 11 and
Microsoft recommended decoder purchased on line (Sonic Cineplayer DVD Decoder Pack 2.3.1).
DVD is not damaged because it plays with no problem on another computer that has Windows 7 OS;
have run Microsoft Security Essential (full screen) & found no viruses.
Computer is a 10 year old Gateway PC - Model 6531GZ.

Symptoms.......
a) 10% of time DVD opens & plays without any problem (no rhyme or reason to successful operation);
b) 90% of time I can hear device attempting to open (spinning) but asks if I want to "Open writable CD folder using Windows Explorer" other useless options like "Burn new CD", "Burn audio CD" or "Take no action". Apparently sees inserted disk as a blank, writable CD - which it is not.

After attempting to resolve this problem for several days, I am ready to give up & purchase a
separate DVD videoplayer for <$100. I need to have a reliable DVD video player so that I can take
packaged DVD video training courses on "personal training "from WITS in the future to maintain my
certification.

Any thoughts/ideas that forum members can suggest would be appreciated.

Happy New Year !!!

Jim D