Many new drivers aren't signed by Microsoft so WinXP will warn you or even prevent you from installing new drivers for a device. To prevent this warning or to allow new un-signed drivers to be installed do the following:
1. Go to start/run/ then type secpol.msc
2. Browse to local policies/security options
3. Change "Devices: Unsigned driver installation behavior" setting to Silently succeed.
4. Reboot Computer.
Another helpful hint from tweakxp.com
Or, especially if you are using XP Home where there is no secpol.msc, right click on your My Computer icon, click properties, change to the Hardware tab, click the "Driver signing" button and click ignore.