Nothing is going wrong as such, Poser will always render at 72dpi, it's just the way it is and unless addressed by a service release will remain so.

If you are rendering for print purposes then you need to do a little maths to work out the final size you need for the print project and then change your resolution in an image manipulation program.

The problems you are having with Poser 7 sound like a graphics card problem, look to see if there is a driver update for your particular card as that will likely fix your problem (just because it is a new PC doesn't mean that it has the latest drivers installed especially if it has been bought "off the shelf" rather than built to spec.