Windows Vista upwards Windows 7 has this built in, and calls this way of positioning a desktop image Fill.
In your example 800x500 would be enlarged to ~1229x768, with the image centered horizontally on your screen, but with an excess of ~102 pixels missing from each side. So, the aspect ratio is unchanged but some image is lost.
Unfortunatly, this feature is not available in XP or Vista, so additional software may be required.
Alternatively, if you don't want extra software, you could manually resize your images so that they match your resolution's aspect ratio. Then using XP's stretch achieves the required results. In your example you could either horizontally trim 800x500 down to ~667x500 (losing some of the image) or add white/black space vertically to make it 800x600.