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Can you take them out and stick them in a desktop or in one of those case to make it into and external hard drive?
Yes.
The desktop option might require a 2.5" -> 3.5" conversion kit though.
For an external case you would of course require a 2.5" caddy.
What Cranberry said.
Physical differences aside (ie, you may need a mounting kit if you don't want it flapping about inside), there's two "formats" of hard disk(*) SATA and the older IDE (2" wide 40-pin / 44-pin data connector). Depending on what you've got, it might need some sort of conversion.
Fitting a 2.5" SATA disk into a SATA desktop is trivial electrically; it's the same connectors. Fitting IDE into IDE will require a converter. Fitting SATA into IDE or IDE into SATA will require more information as to what you've got.
Fitting into a caddy would simply require an IDE or SATA caddy as appropriate. Either way it should then just jack into a USB port.
(* - for the purposes of this question)
Cheers folks, need to open the now dead laptop and find out what type it is