When I switch on my laptop, the router for our home network stops working. It disappears from our available connections list - this affects all devices on our network (other laptops, ipod/iphones etc). Once the router has been rebooted it usually solves the problem, but sometimes it needs rebooted more than once.
I should add that this problem only affects the laptop being switched on from when it was shut down, all other modes do not seem to cause this problem.