I second OpenElec, which is effectively a minimal OS which boots straight to XBMC. I threw it on my aspire revo and it ran beautifully while using under 300mb of HDD. It pulls media wirelessly from my server and I use the android XMBC Remote to control it. I would warn that it probably worked so well for me because they have a build specifically for nvidia ion graphics based machines. Saying that though, they also have builds for Fusion HTPCs, Intel, XTreamer, Apple TV, and a generic build with or without proprietary graphics drivers.