Pc restarts after connecting usb flash key (winxp pro)

Hello everybody!
When i connect usb flash on my pc then windows recognize it and after message “installed..ready to use..” suddenly i get hard reset of the machine! this happens with every usb stick but not happen with digital cameras. I thought that could be power problem so i change the powersupply with a “bigger one” (600w) but still same problem.
Any suggestions?