can you tell me are all the colored messed up ? is it black and white ?
Does your plasma support NTSC

ntsc is all 60hz so does yours support 60hz PAL ?
the first thing i would do is return the xbox HD Adapter you got, sounds to me like the xbox adapter is actually bad.
do you have another device like a dvd player that uses the Red Blue Green cables so you can verify that the CABLES your using are good ? and that your plasma TV red blue green plugs are actually good...

i dont think this is a hard thing to fix because there are only a few things that can be wrong...
MAKE SURE red-red blue-blue green-green in that order red, blue green... i have one of my devices i have to look closely at becasue the blue and the green are very close in color so its hard to tell them apart unless i have a really brite light shining onthem... know what i mean ?
are you running the cables directly to the TV ? or thru some sort of HD A/V video control box like the Pelican HD selector ? dont, plug them directly into the tv.
you MUST try to use something else like a dvd player to use the red blue green and verify that
1) the cables your using are good
2) your tv is ok.. its possible you have a screwed up tv
if your using just cheap cables DONT i have tried a set of el cheapo cables and my video was messed up too.. not like yours but a good set of cables is really a must...
i would say return the xbox hd adapter and get a new one.
last week i got a 3rd party hd adapter off ebay and its junk. the reason i got it is becuase it has HD, S-Video, TOSLINK, and analog video/audio(yellow, red, white) it has all the connections on one unit. but,.. the unit is a piece of crap.. i get MASSIVE shadows on screen when i use the HD connectors.. the colors are drap and the svideo looks like total crap .. but when i switch back to my M$ brand, the real xbox ?HD adapter its an awesome picture.
so something to think about......
lastly.. again if your tv dont support 60hz video then the tv picture is going to look really bad.. so make sure it does support it.