Use Win32 Disk Imager to backup and restore RetroPie SD Images.
Download a local copy of Win32 Disk Imager version 1.0.0 here. Check above link first though as there may be a newer version.
Backup and Restore RetroPie SD Images
Backing up your Raspberry Pi SD Card on Windows
To backup a Raspberry Pi SD Card on Windows use Win32DiskImager.