It's much better to have a separate machine so that you do not have to dual-boot. Failing that, have a completely separate drive.
Some recent machines have the ability to boot from any installed disk-drive, so that you can literally
leave your NT system and
your Linux system completely untouched, and still accomplish "dual booting." You do it by means of your BIOS boot-device selection. And you arrange things so that the two operating-systems basically ignore
But having a separate computer is best, IMHO. Grab that "old" machine out of your closet (which is probably not-so-old, 'cept that the computer salesman on duty that day was really good ...
...) and use it
to learn Linux.