enclosures aren't expensive really
http://www.amazon.co.uk/s/ref=nb_ss_w_h_?url=s earch-alias%3Daps&field-keywords=3.5+drive+enc losure&x=0&y=0
Make sure you get one that matches the drive type (either SATA or IDE... Though with a 500gb it's probably SATA)
But just plugging it in as an internal drive should also be perfectly safe, it will just appear as an extra drive under my computer and work OK complete with the data thats already one it.
if it was going to erase the data it will prompt you "are you sure you wish to format this drive" anyhow