You're going to get a LOT of answers to this one.  My choice of best free tool is the Ultimate Boot CD for Windows.

http://www.ubcd4win.com/

What is the Ultimate Boot CD for Windows?
UBCD4Win is a bootable CD which contains software that allows you to repair, restore, or diagnose almost any computer problem. Our goal is to be the ultimate free hardware and software diagnostic tool. All software included in UBCD4Win are freeware utilities for Windows®. UBCD4Win is based on Bart's PE©. Bart's PE© builds a Windows® "pre-install" environment CD, basically Windows® booted from CD. We include many free hardware and software diagnostic tools that allow you to fix almost any problem you will face with your computer. With network support, the ability to modify NTFS volumes, recover deleted files, create new NTFS volumes, scan hard drives for viruses, etc. this project includes almost everything you need to repair your system problems. This project has been put together to be the ultimate recovery cd and not a replacement OS (Operating System). Please visit the "List of Tools" page for a complete list of what is included in the latest version of the project.

And of course the UBCD http://www.ultimatebootcd.com/

Sysinternals http://technet.microsoft.com/en-us/sysinternals/default.aspx All their stuff is good but you must have Process Explorer at least.

you will also need a test rig back at base to load client drives and  recover data
also a set of malware , virus removals,
plus power supply tester

Two free tools I just downloaded that look good but haven't tested yet are the Avira AntiVir Rescue System and the BitDefender Rescue CD.  Read about them here:

http://windowssecrets.com/2008/08/14/07-Bootable-rescue-CDs-can-fix-your-damaged-Windows#

Another tool I've used in the past is called Emergency Boot CD, but it is no longer free.  You might want to take a look at it anyway.  Then there is BartPE, upon which the Ultimate Boot CD for Windows is based:

http://www.nu2.nu/pebuilder/

What is BartPE and PE Builder?

Bart's PE Builder helps you build a "BartPE" (Bart Preinstalled Environment) bootable Windows CD-Rom or DVD from the original Windows XP or Windows Server 2003 installation/setup CD, very suitable for PC maintenance tasks.

It will give you a complete Win32 environment with network support, a graphical user interface (800x600) and FAT/NTFS/CDFS filesystem support. Very handy for burn-in testing systems with no OS, rescuing files to a network share, virus scan and so on.
This will replace any Dos bootdisk in no time!

Wow!  THERE is a page that immediately goes into my Favorites (which is where I keep a categorized list of all web pages I've found useful about computers and the internet -- several thousand different sites, by the way).  No wonder you're so highly ranked at Experts-Exchange, leew!

Yeah... just gotta get back to updating it... haven't done serious updates in about a year or so...

Here's one more for you leew

http://www.ossmann.com/5-in-1.html

Then there are the times when the PC won't boot and you need to figure out what is wrong.
This is a printable flowchart for troubleshooting hardware problems and the logical steps to take to identify the troublesome hardware device.
http://www.fonerbooks.com/poster.pdf
There are also specific flowcharts links for certain parts that can be accessed here.(they are part of the larger flowchart)
http://www.fonerbooks.com/pcrepair.htm