After a bad crash I re-install the OS and all the individual programs
that were on the disc. I then install files from backups stored on a
slave disc in my PC. Can I make the slave disc a total mirror copy of
the crashed disc (before crash) with OS & programs as well as copied
files and then, in an emergency, simly switch this slave to being
master whilst my sick disc is repaired or replaced?

I think what you are looking for is RAID 1 level protection. Here is a
good article about how to implement RAID protection for a Windows XP
machine: http://www.duxcw.com/digest/Howto/mb/abit/kx7-333/raid-cont.htm

Caution! Read the article carefully before doing anything. Mind you
that this is a technically involved process.

(Windows XP assumed on your computer)

RAID will work but as the other guy said it is a lengthy process and
futile on a home machine...Useful for servers.

I suggest using a disk image. There is alot of software for this:
DISKIMAGE, GHOST, etc;

It would be your best bet.