As a method of promoting system stability, Windows XP attempts to ensure that all drivers installed on your system are digitally signed. Digitally signed drivers are driver files that Microsoft has tested with XP and has certified by adding their digital signature to them. By default, Windows XP will display a warning message every time the installation of an unsigned driver is attempted. In these cases you still have the option of choosing whether you wish to install the unsigned driver, but doing so may impact system stability, so keep that in mind.
The Driver Signing button on the System applet Hardware tab provides access to the Driver Signing window. This tool allows you to configure one of three driver signing options, namely Ignore, Warn, or Block. If the Block option is selected, XP will never allow the installation of an unsigned driver. While this may immediately seem like a good idea, it will lead to issues when attempting to install older software or hardware not designed for XP. In cases where you do need to install legacy hardware or software, the default Warn option is probably your best bet.
Try to avoid selecting the Ignore option; if selected, this setting will automatically install all unsigned drivers without you even being aware that it’s happening. Ignorance may indeed be bliss, but it’s still better to known when XP encounters an unsigned driver, if only for the purpose of troubleshooting issues that may arise later.