To prevent this warning or to allow new un-signed drivers to be installed do the following:
1. Go to start/run/ then type secpol.msc
2. Browse to local policies/security options
3. Change "Devices: Unsigned driver installation behavior" setting to Silently succeed.
4. Reboot Computer.
Or, especially if you are using XP Home where there is no secpol.msc, right click on your My Computer icon, click properties, change to the Hardware tab, click the "Driver signing" button and click ignore.