I have a Gateway FMC-901 that ran MCE 2004. I had to do a complete reinstall of XP Pro, then install the MCE-2005 update that I had. I know many people that had to purchase MCE 2005 directly.
Given that it's an HP, and they did provide the disc, I'm sure someone has it.
Gateway FMC-901 XP Pro MCE 2005
nVidia PureVideo Platinum Decoder Pack
2 gig RAM
120 GB main HD
500 GB secondary drive w/ ripped DVDs
Samsung 46" LCD TV