Standard method: find out what your graphics and sound card are, google '<insert name here> XP driver', verify it's from a trusted source, download, install. If that doesn't work, post the names here.
Though yeah, if it came with Vista and doesn't have well known, widespread components, it may be that they never bothered with XP drivers. In that case, you'd be screwed and need to go back to either 7 or Vista.