I have an old Pentium III, 128MB RAM w/ 80 GB ATA hard drive. Its running Ubuntu, but Ubuntu is a little too user-friendly and i've gone lazy and not really learned much about linux. Enter Slackware...

Ive burned the Slackware ISO's and im at the beginning stages where I would normally use fdisk to to set up the linux partitions. But im a little confused because these partitions already exist due to Ubuntu, I just want to erase all the data. I have two questions:

1) Should I use fdisk to re-partition the drive &

2) How do you do this?

Thanks!

Slackware will offer to let you format the partition as you install it
You can choose a quick format, or a full format, or no format.
If it were me, and it has been a few times, I would do the full format. It isn't absolutely necessary, but I like to start fresh.
Congrats on your decision; I felt the exact same way when I loaded Ubuntu for the first time. Within 2 days, I had Slackware installed and never looked back. I love it.

Also, while you could use something like 'cfdisk /dev/hda1' or something similar, the easiest way to learn about commands like fdisk and cfdisk and parted (this is the command-line version of Gparted, as you would see in ubuntu) is to use the manual pages.
In a console, typing 'man cfdisk' or 'cfdisk --help' will show you the help page for a given function.

So--- On with your Slackware install and remember, it will offer to let you format the partition when you run the installer.
PS - I like Ext2 as a filesystem. You could use one of 2 other formats, but personally, I have had perfect success and ease of use with Ext2.

Great, full re-format it will be! I was thrown off because of all the fdisk warnings/instructions at the beginning.

BTW, what is the difference between fdisk and cdisk? Same thing?

Basically, though they are similar/basically the same, there are lots of varying reports that one or the other is more reliable, and one is somewhat sketchy.. I don't recall which is which so don't quote me, but I think cfdisk is the better one.
You'd get a decent idea by checking the docs for each, and making a comparison. I have seen some threads on the subject around here too.
It may also be the case that cfdisk has more features, but again, I have not very much experience using either.

Great, thanks for your help!

Not at all Welcome to LQ, and to Slackware!

The only major difference between cfdisk and fdisk (as far as I know) is that cfdisk is intended to be a little more user friendly aka easier to use.