Disable Unsigned Driver Dialogue

First go to: Start Run

Then type: gpedit.msc and hit enter. Browse the folder tree to the following location User Configuration Administrative Templates System. 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.