are they all region 4 dvd's? i had an issue very similar to yours as i play region 2 and 4, and there are up to 3 region controls on a pc.. the dvd drive which is easy to get to rpc1, the playback software and the operating system. even though the dvd drive was rpc1 (region free) on mine, it still wouldnt play back the dvd's and MP didnt give any warning about it (dont see why it should either) if it did play it would be hugely pixellated and you could hardly see / hear the dvd.
it may be worth ripping a dvd to hard disk and playing it back that way to see if region control may be the issue. i obviously cant say too much as region control is a dicey issue, but if it turns out to be the case then google is your friend.
by the way Codec packs are the source of all evil! They may seem to be easy and convenient etc, but generally they can screw things up BADLY! i'd suggest sourcing the codecs individually and going from there...
Yeah just wanted to check as its what i think was causing me hassle.. im still not convinced that all is good as half way through the cat in the hat it was stuttering like hell.. and i cant decide if it may be my deregionalisation method, knackered dvd drive or MP..
you'd of thought in this day and age region free would be easier..
Just wanted to mention that I have a similar problem, although I believe it happened in RC1 as well:
When using any codecs except MPV/MPA, DVD playback would start the same as in the OP but then just sit on a black screen. No audio, nothing.
This is using Vista Ultimate 64-bit and EVR. Last night I tried ye olde VMR9 renderer and the damn thing worked fine, except for a hitch in the video every few seconds. This hitch extended to an xvid encoded video as well, so I suspect it's the renderer, but I didn't have time to dig into it anymore.