LiveCD comes to the Rescue
Any professional person needs a good toolset in order to do good work.
In the case of a computer technician, that toolset will include rescue disk/s.
A rescue disk will come in handy when a computer should be accessed but the OS doesn't start, the rescue disk will contain a bootable section, which will be an alternative OS to access the computer and its drives.
A good rescue disk should have a large set of tools which should include anti viruses, partition managers, hardware analyzers and fixers as well as many common tools like a web browser, file explorer etc.
A better rescue disk should have a LiveDisk mode as well as well as a simple windows mode which will contain many tools which can be used in a healthy environment - these tools should be portable and could be used in order to fix a specific problem on a working windows OS.
The livedisk should work with or without an internet connection (which is a problem for antivrus utils as they won't have the latest virus signatures), and the tools will be zipped, and will unzip to temporary memory upon demand.
It should also contain basic drivers for mouse/usb/keyboard etc.
There is a problem with a Livedisk to work using a Windows OS as the windows needs a paid license, thus for many years microsoft tried to block the usage of LiveCDs using their software.
This problem is commonly avoided by technicians using a nice and useful build of Linux, lately being Ubuntu. (As it is updated and has a nice and understandable GUI).
Many of these technicians disks use illegal software, but some of them stay in the light and use free software (or trial tools).
Nowdays, there is no need for floppy diskettes rescue disks anymore so more space can be used for the tools in the disks.
Even CD/DVD rescue disks are outdated as not all computers have a DVD drive, so the best option nowdays is to have a USB flash disk with the rescue disk bootable from it - it will work on 95% of the computers and can have virtually unlimited space on it.
One of the major PC problems is viruses, thus many anti virus companies have their own anti-virus rescue ISOs.
These are good as booting from an external source won't trigger the viruses and enables the tech guy to remove the viruses without them suspecting.
The problem is that you have to hold many bootable flash disks.
So in order to overcome this obstacle I suggest using YUMI which is a free software which enables you to boot to a menu holding and routing you to many boot disks.
I suggest downloading several anti virus rescue disks as well a tech rescue disk or two and bundling all of them in a nice YUMI managed flash disk.
In order to get a nice list of anti virus rescue disk look at this Raymond post
If there is no internet connection on the virus infected PC then you can use this article to download all signatures to those rescue disks and use them offline
Additional to having 3-4 anti virus rescue disk I suggest having a look at the following tech rescue disks (Try these: Bitdefender, Kaspersky, Dr. Web and AVG).
This one is has a dual mode, it can be used as a rescue boot disk using Ubuntu with lots of good software, and it has also a nice windows mode with many tools which can be used (lite version).
All the tools here are legal
It is regularily updated (January 2018)
This tool prides itself for having only one utility for every need (instead of having many different apps doing the same thing).
Its second claim to fame is that it has an automatic mode which will run all tools one after the other and fix all issues (use with caution).
It doesn't install any software on your computer (all portable).
Boots to a fully functional windows 10 (or 7,8 if you pick older dists) which contains lots of installed software on it.
Acutally, I am not sure why I will need this one, as most of these utils are just the regular good day to day software anyone should need, this one is more like a bootable-fully-equipped-windows disk, something which might come in handy for several cases, just not for me.
Latest update was at 04/2017
This is the short history of this tool
Was one of the first
Was the best of its kind
Used illegal software
Updated to all-legal software
Added WinXP mini boot
Added Ubuntu boot
Added portable tools for a working windows OS
Wasn't updated since 2012
This one is only a boot disk, it doesn't work on windows and has no windows utilities on it.
It will boot to a freeDos and holds it's utils in a command like menu.
So if you want to test your hardware without OS interference, or see that these utilities can't be found in the other boot disks, add this one to the list, it is quite small in size so it shouldn't harm your big YUMI flash disk.
It was refreshed at 2017.
Some releases which are not mentioned as they do not suffice IMHO
Active boot disk - They use only their own software
System Rescue CD - Contains only several tools all of which are Unix tools
Trinity Rescue Kit - Latest update was 2016 but even then I think it was minor
Falcon four BootCD - An nice attempt to upgrade Hiren's bootCD, not maintained as it should, but probably better than the non-updated Hiren
Using Yumi install several rescue disks on your flash disk
For Antivirus rescue disks install about 4 good ones (Try these: Bitdefender, Kaspersky, Dr. Web and AVG)
For rescuing/analyzing/fixing install UBCD (For hardware analysis), Falcon four (For older computers this one has it all) and All-In-One for all other cases.
You will also find the utilities from Hiren and AIO-SRT usefull for a working windows OS.
I will also throw in an install of PortableApps with selected apps installed which can come in handy for a working windows OS.
YUMI containing: Bitdefender, Kaspersky, Dr. Web and AVG rescue disks, AIO-SRT, UBCD, Falcon Four.