arduino uno R3 reboot

Hi, i buyed a new Arduino UNO R3 board and i have this problem:

if i plug the usb cable to my netbook with win XP, arduino works with no problem !! If i plug the usb cable to a pc with w7 x64 (the board was installed with no problem using the driver Arduino.inf from the dir arduino-1.0.5\drivers) the board turns on and after XX seconds Arduino reboots !!! Then reboots e reboots etc etc…

I tried a lot of usb ports but nothing !!! i tried to connect also the 12v connector but still reboots… if i disconnect the usb cable, arduino will stay on without reboots!

how can i resolve this ?

The Arduino auto-resets whenever the serial port is accessed.

There could be a program on the PC that is scanning for serial ports.