1. If you want to install Linux onto a machine so that no other system exists, just choose that option at install time; its usually the default.
There is no need to prepare eg format the disk or wipe out another OS; Linux will do it all for you
2. as above, most Linux distributions (aka distros) come with a desktop/wkstn option, which installs a comprehensive GUI over the top of the cmd line.
almost any of the top 10 or 20 are worth a look.
Note also that many offer a LiveCD option, in which Linux runs only from your CD/DVD drive and does not put anything on your hard rive.
This enables you to try it out before installing.
3. dual booting is possible, but takes a little work to setup; a separate machine is simpler:
a) set BIOS to boot from CD/DVD
b) put DVD in drive; boot from Linux DVD + install; remove DVD
4. for a business you may want to look at RHEL https://en.wikipedia.org/wiki/Red_Hat_Enterprise_Linux
if you don't mind paying for updates+support.
Alternatively, Centos https://en.wikipedia.org/wiki/CentOS
is a free re-build of RHEL and includes updates, but no support.
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