I came across this on another forum. Two things to try. Check to see if any of the sticks of Ram are loose - try taking them out and then relocating them one by one.
Another issue could be the power cable. Do you have another one you could try?
Re the speakers - it could be that you need a cable with thicker insulation to cut down on static. Also do you have any other devices such as an aerial, network switches etc? Have you tried connecting the speakers and pc to the same power outlet wit an extension lead?
Re the adapter and internet connection - is your adapter driver up to date? Try connecting by ethernet and then uninstalling the driver in Device Manager, power down and then reboot, letting windows find the driver. Could it be a firewall conflict - the Netgear forum says that there is a problem with some adapters and Zonealarm.