The internal wireless card is an Intel Wireless WiFi Link 5100. The driver used by default is iwlagn. There is mention of iwlwifi being good for injection.

But injection works, to some not-clearly-understood-by-me extent, with the default drivers.

Put wlan0 into monitor mode:

airmon-ng start wlan0 1

The 1 referring to the channel you want to monitor.

Interface   Chipset          Driver
wlan0       Intel 4964/5xxx  iwlagn - [phy0]
                             (monitor mode enabled on mon0)

To list available APs:

aireplay-ng -9 mon0

-9 is an injection test.

In order to go any further, it is necessary to update the standard Slackware wireless drivers with more capable ones. That involves downloading compat-wireless, applying a couple of patches, compiling and installing.

Largely based on Jay Scott:

wget http://www.orbit-lab.org/kernel/compat-wireless-2.6-stable/v2.6.37/compat-wireless-2.6.37-4.tar.bz2
tar -xvjf compat-wireless-2.6.37-4.tar.bz2
cd compat-wireless-2.6.37-4
wget http://patches.aircrack-ng.org/mac80211.compat08082009.wl_frag+ack_v1.patch
patch -p1 < mac80211.compat08082009.wl_frag+ack_v1.patch
wget http://patches.aircrack-ng.org/channel-negative-one-maxim.patch
patch  p1 < channel-negative-one-maxim.patch
make
su
make install
make unload

Now you should be able to follow the entire |WEP crack tutorial on aircrack-ng.org.