Yeah it should work, and the fact that you hit the 5.1 switch and all come on is weird (though it might only be stretching a 2.1 signal to 5.1 as people have said).
One thing that might fix it though:
Go to the Control Panel, and go to Sounds and Audio Devices, and then when the window appears, on it, go to Advanced, under Speaker Settings, and try changing the Speaker Settings to the 6.1 setting or 7.1 setting (if thats all it has) and then press Apply and close the windows off.
Hopefully now, all the speakers will kick in and work (just switch at the back of the speakers to have the full surround again first though)
I hope this helps
Do note though that if you listen to music, unless you have a plugin or program tht will work with 3D sound, you'd get the same output from all speakers.
Check my link on New Audio Player on these forums, and try that out. It has a multi speaker surround setting)
Plus Power DVD can be bought with extra sound features and you can find these in the Sound options of the program, for instance Dolby ProLogic 2 (a worthwhile buy) which splits any stereo music into full surround sound, keeping bass levels, with no problems at all, and of course, Power DVD is one of the best DVD players around!