Try this... With Serial Monitor connected to the Nano, connect and disconnect the other device. Does the reset still occur?
There is an application that is probing serial ports (or USB devices) when a device change notification is sent by the operating system. The usual culprit is a PDA / mobile phone application.As far as I know nothing in the Windows XP operating system behaves that way.
But another possible culprit is the behavior of the RXTXcomm.jar serial driver for Java used by the Arduino IDE.