USB Drivers keep disappear

I submit you a problem with my motherboard ASUS NF7-S 2.0 (WIN XP SP1). Anything I insert in the USB ports – both the ones on the motherboard and those on the extension bracket – disappears after a couple of seconds. I tried to solve the problem by disabling one USB Root Hub after another; disabling the bracket; reinstalling Windows XP on a formatted hard drive; installing device-specific drives, but nothing worked. I have the USB 2.0 drivers and the option to allocate IRQ to USB from Bios is enabled. I can use the USB only for 10-30 seconds, then the peripheral – an external HDD disappears; i have to plug it off from the USB and turn it off before reusing it. The same works for a Flash pen drive, and both have been tested on other computers and found OK an compatible with Win XP.