It is possible to disable XP's check for signed drivers with the code
Of course, it's also possible to disable it manually via Start->Control Panel->System->Hardware->Driver Signing. But the C code from the link above allows an installer to disable it automatically. For the vertical application I maintain, I bundle every possible A/D board driver into an all-in-one InstallShield installation, and I have to use a variety of tricks to make them all install silently. This is the latest such trick.