Slideshow has reduced screen output resolution - why? How to fix?

My Vista PC is connected to a 1080p HD TV with a 1920*1080 panel. The
desktop is set to 1920*1080 resolution and the panel operates with 1:1 pixel
mapping and no overscan or scaling. In normal operation (surfing, editing
photos, etc.) the PC outputs at 1920*1080 to match the TV/monitor. All is
good. However, whenever I play a slideshow, either from the Photo Gallery or
an Explorer Folder the output resolution from the PC is reduced to (I guess)
1366*768 and the slideshow appears within a window centred within the
display with a completely black surround. I know the output resolution has
physically changed because I use remote control software from my laptop to
the PC and when the mouse cursor drags off the edge of the slideshow it
appears back on my laptop, not lost at sea in the black surround on the
display.

If I use other software (Canon Zoombrowser) to output a slideshow I get full
1080 resolution and the display is filled completely. The picture files on
my PC have been sized to match the display perfectly from my 8.2MP
originals, fitting within a 1920*1080 display, so that no further scaling is
required. Why does Vista insist on shrinking my photos further and wasting
loads of screen real estate? There is no point in this behaviour. It is
stupid and unwarranted. Sadly I can find no option to make the slideshow
operate at the full panel resolution. Have I missed something? MCE 2005 was
similarly cack, artificially downsizing photos (probably to 1024*768) and
then expanding them to fill the 1920*1080 screen. This made for some very
unpleasant looking photos. Why does Microsoft have a problem displaying
photos at their correct resolution? How do I correct this defect? There is
certainly no "WOW!" here.