Build for Windows; guidlelines?

Hi,
I've spent the last couple of years building to touch screen tablets, now I'd like to make a PC/Mac version of my latest game and I'm clueless as to how begin prepping the code. I'm aiming to get the game onto an Indie hosting site or similar so the Windows Store and such like isn't too relevant, I'm more concerned with making sure my game works with XP onwards, is the resolution still preferably selectable from menu or can I use a set 1-size-fits-all res? etc...It's not a graphically demanding game at all, so hopefully most machines will run it perfectly.
If anyone has any guidelines to this or pointers to sites, particularly to Unity resolution setting with such info I'd be very grateful.