Melissa,
Here's what i would do:
Start your presentation and show the slide you want as your desktop
background. Press PrtScreen button on your keyboard. Now go to any graphical
program you have on your computer, if need be this could be Paint.
Personally i use Irfanview. Paste your printscreen in, save as a picture
(jpg, png...)
Now on your desktop, rightclick, choose properties, choose tab desktop,
click on browse, choose your picture open it, and hit OK. The picture should
fill your entire screen.