As Zach already implied, it's no problem to take out your internal harddrive and put it into a external enclosure. I've done so in the past several times, when I bought larger capacity internal drives and put the older smaller ones in external enclosures.
Just make sure you got the right type and size enclosure. If the HD is a few years old, it's likely to be IDE, so don't get an external enclosure that only supports SATA drives, etc.