The reason for the delay is pretty weird, Microsoft states some obscure content error which includes partial nudity was included in the initial production of Halo 2 for Windows Vista.
So what exactly is in there? We don't know. What counts as an obscure content error? We're not sure there, either. What is Microsoft going to do? Well, try to avoid any legal shenanigans that's for sure - retailers have received stickers to put on Halo 2 packaging to warn about the content, which can be removed with a patch downloaded from Microsoft.