hello everyone I have a slight problem. how to attach a second hard drive into this pc and have it act as the slave?

I have purchased a secondhand pc for my son ( to upgrade his old one)
I want to transfer his Hard drive from his old pc (120g) to the new pc which only has 40g hard drive. thereby making it 160g HDD
when i opened the case the harddrive was screwed onto the case front and not in any bracket system.
How can I install the old HDD? Are there special brackets for this? And if so where would i purchase them.

i want to do this so I dont erase the recovery partition on the new HDD and also so my son does not have to erase all his private data  on his  old HDD