In Windows XP, there is a warning box whenever you try to install drives that are unsigned. Many times you can just ignore the warning and install the driver anyway, but sometimes you are just stuck right there. There is a way to disable these messages:

(1) Click Start > Run.
(2) Type “gpedit.msc” and hit OK.
(3) Browse to User Configuration – Administrative Templates – System.
(4) Right-click Code signing for Device drivers and select Properties.
(5) On the Settings tab, either select enable, and then select ignore from the appearing listbox…, or click the disable option.
(6) Click Apply then OK.

Alternatively, you can go to Control Panel > System. Then go to the HArdware tab and click on Driver Signing. Choose Ignore in thre resulting window.