Media Portal is very nice. I have used it quite abit. It is a project by the lead guy who did XBMC, which is GREAT.
However, if you need an OS anyways, and want to go legit, I would still recommend MCE. It is nice that it comes up-to-date with SP2, and even though MediaPortal is great, I still prefer the intrergration of the media center. It works great with the remote and is just a little more seemless than running mediaportal.
If you aready have XP, then yeah, I would just add on MediaPortal.