It's not as simple as that. I found a sysoc.inf for MCE inside the SP3 package (it's named sysocmce.inf and it's in the i386 folder). Editing TXTSETUP.SIF and DOSNET.INF is easy, and adding extra hives to allow installation of the MCE-specific programs shouldn't be too hard either. But that's where the good news ends. There are 20 new Media Center binaries (not "updated", but "new" binaries) for the MEDIACTR.CAB package, so a source HFSLIP can work with can't be created unless HFSLIP were to rewrite medctroc.inf completely. This would require a lot of code, and languages with double-byte characters wouldn't be supported. The updated Media Center binaries require .NET Framework 1.1, which is not included in SP3. So they can't be slipstreamed, actually. This is the reason why HFSLIP doesn't slipstream the *MediaCenter2005*.exe updates. MS could've copied the OC install version of .NET 1.1 from Server 2003 but they haven't. Slipstreaming SP3 replaces existing WMP10 binaries with WMP9 binaries. It will take time to find out which ones are to be restored, and how.