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
*www.mydigitallife.info/2007/01/19/download-free-vista-codec-package-423/
for windows xp x64 :
*majorgeeks.com/downloads32.html
try seeing till end
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*www.softpedia.com/get/Multimedia/Video/Codec-Packs-Video-Codecs/Advanced-Vista-Codec-Package.shtml
*www.digital-digest.com/software/Vista_Codec_Package.html