Swapable Hard Drive

I am technician in a computer repair dept. and i REALLY need an OS that I can both understand (Ubuntu scares me) and can be moved to different machines. I constantly get computers in that work but the owners have screwed around with their settings and functions so bad it won't work any more. If I could just pop in the HDD and Voila it all works then Bob's your Uncle the hard drive can be wiped and I could go on my merry way to another machine.

Oh by the way I know about Windows PE---the problem is I can not download drivers to PE and test all the functions this is the CLINCHER!!

I used to do this all the time with Windows 95 and 98 but XP and Vista just put me in the reboot cycle and the fun never ends!
Sorry I am such a Noob but things have changed since I originally started working on PCs and then I took a break for a couple of years and now I can get lost.

any suggestions would be appreciated.
:major