In theory, you 'should' be able to a previously internal hard drive working in an external hard drive case (complete with power supply and cables).
That said, never tried it with a laptop hard drive before - so check with the vendor of the external hard drive case - make sure it's complete with all cables. May need special adapters (internally) for hooking up to smaller laptop drive.
Next questions are easy - you won't have problems transferring files from one disc to another - it's immaterial what operating system was resident on your external drive.
Will it be a usb external device?
You won't be booting up from your old Win2000 disk, but you should get your files off of it.
You won't be able to use the software that was on it. You'll need the full installers to install the software on the new internal drive. Don't be tempted to use pirated software - it may be full of spyware etc..
If you can't afford legitimate software, give open source alternatives a try - e.g open office instead of Microsoft Office, available here:
www.openoffice.org
There's a wealth of quality software out there, not just from commercial vendors
Good luck - main advice, check with vendor of your external hard drive casing regarding fitting laptop drive
DaveB - LinuxWannabee