Computer boots when I switch off the lights

I have more details regarding My computer boots up during the night. I got a used halogen lamp with a transformer and can reliably reproduce the following:

• Computer and lamp are ruing
• I shut down Windows until the computer is "off".
• I switch off the lamp.
• The computer starts its boot cycle.
• I press power switch to shut down computer.
• The computer stays off until I hit the power button again.

I am not really sure what the reason is and what to do about it. Any ideas?