I haven’t played with XP Service Pack 2 yet due to not having ready access to a Windows smachine that I can upgrade, but it seems that one of the security changes made to IE has greatly increased the importance of serving up the correct Content-Type header. Aldo Hoebon writes about this in HTC components in XP Service Pack 2, and describes how the wrong Content-Type for a proprietary IE HTC file (as seen in the alpha-transparent PNG fix for that browser) can cause the HTC file to have no effect.
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