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.
" Windows XP Tips: Disable Unsigned Driver Dialogue "
Reference : techpraveen.com