XP Pro: Start - Run gpedit.msc

Browse to:

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.

XP Home:

Open "System" properties and select the Hardware tab.

Choose the Driver Signing button and choose "Ignore" to install the new driver anyway.