I compressed everything down to it's basics. Just type this into Notepad. Save it with "save as text" if there are multiple save options in Win-XP. Duplicate the image files and rename them with these shorter names (for easier typing).

vSD = ImageReader( "SD360.jpg" )
vHD = ImageReader( "HD720.jpg" )
vSD
#vHD

That's all of it. This script will display the SD image. To display the HD image, put a # in front of the vSD and remove the # from #vHD.

This must work (it does work for me). After that, you must see what could be wrong in your copy-and-paste. That problem will stop you from using Avisynth.