System Freezes at POSTwhen USB Ext Drive ON

I have noticed that on some systems running WinXP SP2 that the system freezes at the inital stages of POST when an external USB HDD is on. If the external USB drive is powered off, the system boots normally. I also noticed the same problem with a USB printer, which when on produces the same result (system freezes at initial stages of POST,) but when the printer is powered off the system boots up normally. Any suggestions?