This option wll disable the screen wich keeps popping up when you try to install 'digitally unsigned drivers'.
Normally you can choose to continue the install anyways, but I have had situations where you cannot continue the install.. very annoying..

This is how to fix it.

Click Start - Run
then type: gpedit.msc
then hit enter.
Browse the folder tree to the following location:
User Configuration - Administrative Templates - System

now right-click Code signing for Device drivers and select Properties.
On the Settings tab, either select
- enable, and then select ignore from the appearing listbox..
- or click the disable option.

Click apply and Ok and your set!

Alternatively especially for XP Home users:

Open "System" properties (Windows key + pause or Right click 'My Computer' - properties or Control Panel - System).
On the Hardware tab click the "Driver Signing" button. In the dialogue that comes up choose "Ignore" to install the new driver anyway.