You must create floppies from the bootdisk and rootdisk images that you downloaded (or have on CD-ROM), no matter what type of installation you will be doing.
On an MS-DOS system, you must uncompress the bootdisk and rootdisk images using GZIP.EXE. For example, if you're using the bare.gz bootdisk image, issue the MS-DOS command:
C:\> GZIP -D BARE.GZ
which will uncompress bare.gz and leave you with the file bare. If you are installing from CD-ROM, you can copy the bootdisk image (such as bare.gz) to you hard drive, and run GZIP.EXE from the CD-ROM to uncompress it.
You must similarly uncompress the rootdisk image. For example, if you are using the rootdisk color144.gz, issue the command:
C:\> GZIP -D COLOR144.GZ
which will uncompress the file and leave you with color144.
Next, you must have two high-density MS-DOS formatted floppies. (They must be of the same type; that is, if your boot floppy drive is a 3.5" drive, both floppies must be high-density 3.5" disks.) You will use RAWRITE.EXE to write the boot and rootdisk images to the floppies.
Issue the command:
C:\> RAWRITE
Answer the prompts for the name of the file to write (such as bare, or color144) and the floppy to write it to (such as A:). RAWRITE will copy the file, block-by-block, directly to the floppy. Also use RAWRITE for the root disk image. When you're done, you'll have two floppies: one containing the boot disk, the other containing the root disk. Note that these two floppies will no longer be readable by MS-DOS (they are ``Linux format'' floppies, in some sense).
Be sure that you're using brand-new, error-free floppies. The floppies must have no bad blocks on them.
Note that you do not need to be running MS-DOS in order to install Slackware. However, running MS-DOS makes it easier to create the boot and root floppies, and it makes it easier to install the software (as you can install directly from an MS-DOS partition on your system). If you are not running MS-DOS on your system, you can use someone else's MS-DOS system just to create the floppies, and install from there.
It is not necessary to use GZIP.EXE and RAWRITE.EXE under MS-DOS to create the boot and root floppies, either. You can use the gzip and dd commands on a UNIX system to do the same job. (For this, you will need a UNIX workstation with a floppy drive, of course.) For example, on a Sun workstation with the floppy drive on device /dev/rfd0, you can use the commands:
$ gunzip bare.gz
$ dd if=bare of=/dev/rfd0 obs=18k
You must provide the appropriate block size argument (the obs argument) on some workstations (e.g., Suns) or this will fail. If you have problems the man page for dd will be instructive.