By default, the backup folder is on the sd card. If your device has a removable card, you can just put it into your pc & access it. You can also access it through USB by using adb commands, but you must have previously turned on debugging mode in the settings.
It's also handy to have AROMA Filemanager
on your sd card. You flash it in recovery like any other zip (it doesn't make any changes, just launches the file manager). It also comes with a terminal emulator. It's quite powerful, and extremely handy when you can't get your system to boot up.
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> 10 Nandroid backup
> 20 Flash experimental .zip
> 30 IF (works) THEN GOTO 50
> 40 Nandroid restore
> 50 Eat cake