I think this is controlled by your printer. From the IE7 FAQ page:
" Why do I have to reset the print options every time I open Internet Explorer?
Printing changes made within Internet Explorer are always temporary. To make changes permanently, you need to change them in Windows. For information about changing Windows print options, see Choosing print options."
I didn't see anything in that link particularly useful though ... and when I tried to simply force my printer to print at 100% through the print driver settings, it would always reset itself to Actual Size. Worth a shot for you perhaps, via Properties > Advanced > Printing Defaults > Effects (but that's when using my HP LaserJet 2420 ... will be different depending on make and model of printer, and some printers may not even support this option)
You may need to contact your printer manufacturer if it's bugging you that much, see whether your printer can handle it