The 3.5" ones are for standard desktop hard drives. 2.5" is the standard for notebook drives. You just have to make sure you get one with the right connector, IDE or SATA. I have several Bytec IDE enclosures for old drives, they work fine.
Ok, here is what you do. Unscrew it from your case... and set it on your desk.
DONE, its now an external harddrive!
Ouch, that was cold
whats a IDE and a SATA connector??
how can i tell what i have?
how does a IDE cennector look like and how does a SATA cennector look like??
thanks you for answering
Okay, the "genius" nickname has to go. An IDE uses a huge 40 pin connector, a SATA is very tiny by comparison. You can google (should I say www.google.com ? nah....) for pictures of what each looks like. If you bought your computer in the last couple years, it almost has to be a SATA. But you really should check your computer's documentation to be sure you get it right.
And as someone mentioned, once you place it in an enclosure, the connection will be Network, USB, or Firewire. Network can be a little tricky, USB and Firewire should literally be plug and play simple in any recent OS (although Firewire loves to give me problems in XP, but thats just me).
I have to agree that Genius handle is going to get you in trouble really quick asking questions like this.
No question is a dumb question, but never choose a self absorbing name to identify yourself with... lol. You will have to douse you're self in non-flamable liquid to stop the flames from licking you....