You say "as it logs on" - do you mean as soon as it has booted into Windows, or when it's establishing a connection (or trying to) with the router? My first thoughts are some kind of spurious RF emissions from the power supply. Are we talking of a laptop or desktop? If a laptop, what happens if you move it further away - the next room for instance? Failing that, what happens if you disable the wireless card? And does the computer also have Bluetooth built in and operating?