Is it plugged into a phone line? I had the same issue a few years ago, it turned out that the Telco were testing lines and Wake On Ring on the modem was set, firing up the pc.
More modern, check the BIOS and see if "Wake On Lan" is set. Turn this off and see what happens. If it does stop, you then have to work out how these "magic packets" are getting there.
Only the truly stupid believe they know everything.
Stu.. 2004