I suggest getting the install.1 and install.2 images as rootdisks.
The installation with bare.i, install.1, and install.2 (in that order) goes pretty smoothly on whatever machine I try it. After the install.2 one you get to put your Slack CD in, at which point all the nifty packages can be selected etc.
P.S.: I just thought of this, but I'm not quite sure if it actually works... try pressing the ESC button right as your computer loads (during like the very first image you see); supposedly it will give you a menu of boot locations (including, hopefully, the CDROM drive). It didn't work when I tried it, but you know... it can't hurt to try