oh I just checked another prg I have - it's free too!
Photoscape
in the edit section when you save a picture - at the bottom of the pop up window there is a DPI setting - you need to check the box and then you can set it for 300 dpi.
It has a lot of settings and easy to get over whelmed but there is an undo button and I always save as a "new name" so I always keep the original and it doesn't get changed.
get it here-
http://download.cnet.com/PhotoScape/...html?tag=mncol