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I can understand your caution about removing the original hard drive. What type of drives, IDE or SATA?
If IDE then you need to make that new drive a 'Slave' since your existing drive is probably configured as 'Master/Single'.
If the drives are SATA then just place the drive on the proper connector on your motherboard.
To use the Slackware Install cd1/DVD you can now boot with either the default kernel 'hugesmp.s' by pressing enter or select the 'huge.s' kernel. Both of these kernels are installer kernels. The generic kernels are recommended for daily use.
You should read the Slackware 12.2 CHANGES_AND_HINTS.TXT
Once you have booted into the installation you should now be able to configure your new drive as desired. Once you have done with the install you will be asked about installing 'lilo'. 'lilo' is a bootloader that is used by Slackware. You can either write the bootloader to your 'MBR' of the master or to the superblock of your new install. With the 'MBR' and you have selected the proper information for your 'Vista' then you would be able to select the desired OS at boot.
You could configure the M$ to control your booting but I prefer the open source method(s).
If you select to write to the superblock then you would need to use something to allow you to boot the Slackware OS. You could use 'USB flash' or the install media then pass the proper information to the kernel at boot. If you read the boot screen when you use the install media there is a good example that you can use with your configuration information to boot.
You could do a search here on LQ for a Slackware install with a Vista machine. I know this has been covered many times.
'Vista SP1 won't install on dual-boot systems
' is a must read if you are using SP1.
'Dual/Multi Booting With Vista
' is another good reference.
There are several other good links in the 'Get SlackwareŽ Linux
' section of 'Slackware-Links
'. More than just SlackwareŽ links!