Check your bios for what is allowed to wake the computer - e.g.wake-on-lan, wake-on-mouse.
After disabling all except what you want to wake the PC (just the keyboard I would assume), then you go into control panel, power settings, and also some of the hardware in device manager. Network adapter properties, for instance, has a power tab checkbox setting 'allow this device to bring the computer out of standby'. Should be blanked because your broadband is permanently connected and receiving keep-alive signals.
There is a theory that certain MS or motherboard updates can reset certain options to default, which is not what you want to happen.
Mouse, modem (if on the motherboard) and keyboard also have that power-saving tab with a wake-up checkbox.
Captaincranky points out that radio interference can affect your PC from a distance, so maybe your neighbour has just got some new toy/radio/tv/mobile that is leaking a lot of RF power.