IrfanView should be on everyone's PC. It's a superb image viewing program but it can also be used for image manipulation:
http://www.irfanview.com/
To change the dpi of an image file, load the file and go to Image > Information. Enter the desired resolution, click 'Set' and 'OK'. Then go to File > Save as. (Don't use 'Save' unless you wish to over-write the original file).
Chris



