All desktop computers use 3.5" disks.
However hard disks can have two types of connectors at the back.
The older type (say more than 5 or 6 years old) have IDE connectors.
Newer disks have SATA connectors.
You need to buy the right caddy for the right disks (SOME caddies may do both).
The connectors on an IDE are rather large and clunky, those on a SATA are small and delicate.
Here is a picture showing both (note the bottom SATA hard disk shows it having an old MOLEX power connector, but most SATA drives dont have this)
http://pc-level.com/w...ide-hard-drives-1.jpg