I use a business class HP Compaq USDT (Ultra Slim DeskTop) with a built in GPU (Think it's like a Radeon 4500 series? The PC is a few years old) and I have it running Windows 7 and hard wired to my network. I have played with XBMC and Plex and both have worked just fine. However, I find myself more often than not just using VLC Media Player (I have the Remote App on my phone and iPad). I have a Windows Server 2012 Essentials Server running in my house that has all of my network shares for music, installs, pictures, videos, recorded tv, etc... and just stream everything over my network from that. You obviously don't need all that, just share out your videos from your PC/Laptop and play them on the HTPC.
Long story short, you don't need much for an HTPC just something small that can play network shared videos on your TV.