Unless these machines are on a domain, you cannot easily mass configure your Windows XP machines to dismiss this dialog. That dialog is there for a reason: To prevent drivers from silently installing and undermining your OS's stability and security.
If determined, you could perhaps develop a script that simulates a user performing the following reconfiguration steps:
- On the desktop, right-click My Computer, and then click Properties.
- Click the Hardware tab, and then click Driver Signing in the Drivers area.
- In the What action do you want Windows to take? area, click the desired action, and then click OK two times.
For more information, check out Microsoft Support article 298503 entitled "Driver signing registry values cannot be modified directly in Windows".