Simply put your card with your existing installation in to a USB card reader and get the correct drive letter from Windows Explorer. Make sure the correct drive letter is selected in WinDisk32. Give the image a name like RetroPie_Backup_4-2-2015.img. Click read.
After waiting what can seem like an eternity, you will have your backup image.
To put your backup image on a card, the process is the same as writing the initial RetroPie image. Put your clean card in the card reader. Make sure you select the correct drive letter. Select your RetroPie_Backup_4-2-2015.img. Click write.
Wait another eternity and you should have a copy of your original installation.