maybe you cannot.
you would need to open the enclosure and remove the drive. the drive may be soldered to the control board and if so you cannot, sorry.
if it has a standard SATA interface you can simply remove it and install the disk as you would any other drive.
change the boot order in the BIOS and install the OS to the 500GB.
if the drive has an IDE interface you will need to change the master slave jumpers on each drive or set them to cable select. then connect both drive to the ribbon cable and verify the boot order and go.
Canned Grounding Rant-
shut down system and remove side panel. with the power cable plugged into the PSU touch a bare unpainted metal area of the case. (my favorite spot is an unpainted screw securing the PSU) once you have grounded yourself you can unplug the computers power cable from the PSU and can touch the system.
if you move your feet, or shuffle in your chair, plug in the cord, reground yourself and unplug again.
end canned rant-