iso file with view to install trial operating system

There are two ways:
1) Burn the ISO to a CD/DVD (depending on size) and install the operating system using the media you burned (booting your system from CD/DVD drive)
2) As Ded9 said install a virtual environment, I would suggest Oracle Virtual box ( https://www.virtualbox.org/ ) where you can create a Virtual machine on your existing system and install the OS using ISO you have downloaded.

Download Virtualbox: https://www.virtualbox.org/wiki/Downloads
Documentation : https://www.virtualbox.org/wiki/End-user_documentation

Regards,
Maxmk

Apart from what has already been mentioned above, many Linux OS's that come as iso files are actually a LiveCD/DVD, which means that is you want to just do some testing all you need is to boot from the CD/DVD you burnt (or if that OS is supported, put it on a USB stick by using the tool in the Link lower down, and boot from that, which is faster than a CD or DVD). You then get into the OS's environment without needing an installation at all and you can run your tests and find out whether the OS could be something for you. This is very useful if you want to test several OS's before you decide which to install. Those Distributions usually have an installer which is displayed on it's desktop.

http://unetbootin.sourceforge.net

Hi I am not familiar with virtualization so will use DVD method first but will also try virtualbox as excercise. Is virtualbox fairly intuitive though?

Will presumably need partition for dvd? I have allowed 100 gb

It depends on the OS. What OS is it? As I mentioned, many linux distro's come as LiveDVD's and don't need to actually be installed if you just want to test it. If you do install it, then the space needed also depends on the OS. Again most Linux OS's don't need a lot (usually 10 to 20GB is more than enough).

Having a go at Windows 2012 server essentials

According to the specs of that OS you need at least 160GB disk space.

Apart from that in my point of view particularly for Windows 2012 or 8, they use a newer bootloader than Windows 7 does, and that can be a little tricky to get rid of when your tests are finished. For this reason I suggest you get an extra harddisk for this test, and when you install this OS remove the other HD's from your system. Just replace your disks whenever you need to switch the OS.

rindi

or others

does the possible problem with bootloaders arise with a virtual disk ?

Hmmm, I never thought of this before these last two comments - but another benefit to VHD boot is that your machine does not need the power to run a Host (on real HW) OS, the virtualization software and have enough resources to spare to run the additional OS.

i have 6 gb ram and 64 bit system.

the reader comments generated from the zednet link above from CSI-windows seem to imply/state that booting from VHD is only poss with Windows 7 Ultimate and not windows home premium (which is what I have)

Would this be a correct interpretation of that article?

Besides burning it to CD/DVD, you could also copy it to a bootable USB stick... depending on what OS it is, some of them even have programs especially for putting their own images onto USB sticks (e.g. microsoft's USB/DVD tool, fedora's USB creator, et al).

Yes you'd need the Ultimate version.

final question...

if I unzip the files to my very fast external hard drive, will I be able to boot from there (with a bios twist) and test from the external drive.