How to disable my netbook's touchpad when a usb mouse is connected

It depends on the driver you have, if it is an Synaptics touchpad, you can disable it by right clicking on the taskbar icon.

For Alps, I think you can do it from the mouse properties in Control Panel.

For anything else, or different versions, just hunt around a mix of the above - either the mouse options or taskbar icon.

Click Synaptics tray icon, select "Pointing Device Properties"; select "Device Settings"; you can either disable it, or, in Windows 7, have it disable automatically if you connect an external mouse.