The first thing you need is some non-partitioned
space in your harddisk. Harddisks are divided into slices called "partitions". Windows is used to create only one slice that uses all the harddisk. If that's your case, then you need to use some tool to resize it, leaving space free for creating other slice to put linux in.
What Windows do you have? The filesystem is the way your files are stored in a partition. This is different from Windows to Linux. For instance, Windows XP can use FAT32 and NTFS filesystems. As you want to install Linux from harddisk, you should check that a) either your Windows is using FAT32 or b) either the Slackware installation program is able to read NTFS filesystems. If neither a) nor b) are true, then you will need to create a second partition with FAT32 (Windows will allow you to format it) and put your Slackware files there.
You will need to create boot disks. I'm not a Slackware user, so I cannot help you too much with this. Nevertheless, I'm sure that it is possible and the Slackware webpage will tell you how.
Maybe you have done all this already and you are only asking how to mount, but I preferred to check first...
Now, Windows names your partitions with letters (A, B, C). Ussually A and B are for floppies, C for harddisk and D for CDROM, but if you have more than 1 harddisk partition, the second will be D, and then the harddisk could be E, etc, etc. We could say that the partitions are "mounted" in these letters, as under these letters is where you will find them. In linux the system is different. There is only one "unit", the "root", which is symbolized as "/". You can imagine that "/" corresponds to "C:\". Anything else (even if it is in other partition) will be under this structure, so you have to "mount" them. This is done in a directory. The directory you want! But usually, what people would do is go to the "/mnt" directory (this is done with "cd /mnt") and create a "windows" folder (run "mkdir windows"). So this will be place where the windows partition will stay (=be mounted). However, you still have to MOUNT it (you could call "clown" to the folder. Naming it "windows" means nothing for Linux). This is done with the "mount" command, which is used as follows.
mount -t <filesystem> <what do you want to mount> <where do you want to mount it>
As you can imagine, "<filesystem>" and "<what do you want to mount>" will depend on your partitions and if Slackware is able to read NTFS systems or not. The most optimistic thing is that Slackware is able to read NTFS and you had only one partition. Then you had to run
mount -t ntfs /dev/hda1 /mnt/windows
On the other hand, if Slackware wasn't able to read NTFS and you had to create a second partition, then you will need to run
mount -t vfat /dev/hda2 /mnt/windows
But there are a lot of others possibilities... Anyway, I hope this could help you a little...
PS: Do you have any particular reason for using Slackware 9.1? I ask you because a) Slackware is in version 10.1 now. b) There are some Linux distros that can be installed from a USB-Key.
PS2: You asked for plain english. I hope mine was "plain enough" or "good enough", as I'm not an english native speaker.