As far as downloaded digital games are concerned, one just needs to plug in a USB drive and copy/move the file from the internal HDD of the Xbox 360.
Then you one just needs plug in that same USB drive to a Windows PC (tested on Win. 7) and just copy and paste the files to the desired backup location.
The formatting was found to be,
<USB drive letter>\Content\0000000000000000\<Title ID>,
where the Title ID correspond to either the main games
or XBLA games
As far as games ripped from the disks are concerned, it was not possible to transfer from the 360's HDD to the USB drive.
Its likely that the disk has to rip directly to the USB drive in the first place for this kind of backup to work.