It is a case that is used to turn an internal drive into a external (more portable?) drive. Something like this
https://www.amazon.com/Sabrent-Exter...hard+disk+case
The only other question is PATA (wide, flat cable. also called IDE) or SATA (narrow cable)
Since you said 'Tower' You most likely have a 3.5" drive (2.5" are in Laptops and some rack servers).
The adapter is for more temporary use, like cloning or transferring contents.
This one works with many interfaces:
https://www.amazon.com/dp/B01NAUIA6G...d-0d6a86c107a2
You don't have a neighborhood nerd kid handy? I am not a Worst Buy fan, but then I have options (and do this kind of stuff myself)