Hi Dabster !!!
i think not many are using X64 editions as they are hard to get specially i got X64 Xp in 2006 may that also i know how !!! but yes the problem you are facing are related to the copyrighted codecs.. are you using the full version of Win XP X64 or eval.
i can get you the codecs for x64 windows.. but you got to wait for two days .. i downloaded these a long time back but i forgot the link i downloaded those from.... i will let you know as soon as possible or (ASAP)

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for windows vista X64
http://www.mydigitallife.info/2007/0...c-package-423/
for windows xp x64 :
http://majorgeeks.com/downloads32.html
try seeing till end
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http://www.softpedia.com/get/Multime...-Package.shtml
http://www.digital-digest.com/softwa...c_Package.html