I have partitioned my disk to boot into Xubuntu and Slackware as so:

hda 1Gb swap
hda2 40Gb linux
hda3 19Gb linux

However when I log in to Xubuntu the 19Gb partition for Slackware appears as an icon on my desktop and within thunar.

It is not mounted. Is there anyway to get it not to load into the system?

Its quiet easy to edit fstab. See if there is a entry in /etc/fstab regarding the /dev/hda3 . If so remove the entry & see if this solves the problem. Read the man pages of fstab for howto add a entry, in case later you feel you want to share the drive with both xubuntu & slackware. If that doesn't solve the problem posting the fastab here would be usefull.

I have a spare box running Xubuntu (to try out Xfce), & I don't see the behavior/problem you speak of -- the partition set aside for Etch does not show up on the desktop or in Thunar.

Based on my (much more extensive) experience w/ KDE, I think this is a configuration issue & not controlled by fstab. If you were running KDE, & I know you're not, then right clicking on an empty part of the desktop would start you on the way to configuring it to not display unmount partitions. Unfortunately, I am too new at Xfce to tell you where to find the equivalent setting.