to make it nice and simple...
turn the e260 player off (hold power button), after it's off...
connect the USB cable to the e260 and then plug in the USB cable (windows will detect it and install a driver etc for it (if your using it for the first time)) this will automatically make the player boot into the original sansa firmware and not rockbox as the bootloader can detect the presence of the usb cable and know to boot into original sansa firmware.
then load up windows explorer (i.e. my computer etc) and find the e260 in windows explorer and find music on your pc and then copy and paste it to the e260.
that's that, have fun