What's probably happening is this:

• Your PC has a clever feature usually found on servers, but also in several desktops, in the setup, saying something to the tune of: "In case of power failure: [ ] do nothing [ ] stay off [X] last state".
• Your PC's power unit is slightly faulty - probably a capacitor is a bit cooked. When it feels a sudden "spike" in voltage, such as you get when you switch an inductive load, it considers this as a temporary blackout. Therefore it resumes its previous state.
• Windows does not set correctly the "off" state (this is a common APM problem), i.e., the low level circuitry considers itself to still be on. Possibly this is because the desktop is partially on, e.g. to support wake-on-lan.
• when you turn the PC off from the button, it goes to "really off", so when it senses another blackout, it stays off.

You can break this cycle in several ways:

• use some spike suppressor (anywhere from USD 1,99 to USD 19,99) on the PC's power outlet. Note that they're power rated, i.e. if your PC absorbs 700W, you'd better use a suppressor rated at least 1kW, or it can either not work, or positively malfunction. This will probably stop the problem from occurring, whatever the real cause, by simply "hiding" the external spike from the PSU.
• disable "Power Resume" and/or "Wake on LAN" functions in the BIOS.
• there are several voodoo rituals that may make Windows convince the PC that it's lights out, really.
• replace the PSU. Shouldn't be needed, but this behaviour might be symptomatic of something worse coming. If you've got a slowly cooking capacitor, it may pop at some inconvenient time (they usually do).
• set the "In case of power failure" to "[x] STAY OFF!" if at all possible.

What's probably happening is this:

• Your PC has a clever feature usually found on servers, but also in several desktops, in the setup, saying something to the tune of: "In case of power failure: [ ] do nothing [ ] stay off [X] last state".
• Your PC's power unit is slightly faulty - probably a capacitor is a bit cooked. When it feels a sudden "spike" in voltage, such as you get when you switch an inductive load, it considers this as a temporary blackout. Therefore it resumes its previous state.
• Windows does not set correctly the "off" state (this is a common APM problem), i.e., the low level circuitry considers itself to still be on. Possibly this is because the desktop is partially on, e.g. to support wake-on-lan.
• when you turn the PC off from the button, it goes to "really off", so even if you switch the lamp again, even if it senses another "blackout", it stays off. That's why the lamp-switching only works once.

You can break this cycle in several ways:

• use some spike suppressor (anywhere from USD 1,99 to USD 19,99) on the PC's power outlet. Note that they're power rated, i.e. if your PC absorbs 700W, you'd better use a suppressor rated at least 1kW, or it can either not work, or positively malfunction. This will probably stop the problem from occurring, whatever the real cause, by simply "hiding" the external spike from the PSU.
• disable "Power Resume" and/or "Wake on LAN" functions in the BIOS.
• there are several voodoo rituals that may make Windows convince the PC that it's lights out, really.
• replace the PSU. Shouldn't be needed, but this behaviour might be symptomatic of something worse coming. If you've got a slowly cooking capacitor, it may pop at some inconvenient time (they usually do).
• set the "In case of power failure" to "[x] STAY OFF!" if at all possible.

What's probably happening is this:

• Your PC has a clever feature usually found on servers, but also in several desktops, in the setup, saying something to the tune of: "In case of power failure: [ ] do nothing [ ] stay off [X] last state".
• Your PC's power unit is slightly faulty - probably a capacitor is a bit cooked. When it feels a sudden "spike" in voltage, such as you get when you switch an inductive load, it considers this as a temporary blackout. Therefore it resumes its previous state.
• Windows does not set correctly the "off" state (this is a common APM problem), i.e., the low level circuitry considers itself to still be on. Possibly this is because the desktop is partially on, e.g. to support wake-on-lan.
• when you turn the PC off from the button, it goes to "really off", so when it senses another blackout, it stays off.

You can break this cycle in several ways:

• use some spike suppressor (anywhere from USD 1,99 to USD 19,99) on the PC's power outlet. Note that they're power rated, i.e. if your PC absorbs 700W, you'd better use a suppressor rated at least 1kW, or it can either not work, or positively malfunction.
• disable "Power Resume" and/or "Wake on LAN" functions in the BIOS.
• there are several voodoo rituals that may make Windows convince the PC that it's lights out, really.
• replace the PSU. Shouldn't be needed, but this behaviour might be symptomatic of something worse coming. If you've got a slowly cooking capacitor, it may pop at some inconvenient time (they usually do).
• set the "In case of power failure" to "[x] STAY OFF!" if at all possible.

What's probably happening is this:

• Your PC has a clever feature usually found on servers, but also in several desktops, in the setup, saying something to the tune of: "In case of power failure: [ ] do nothing [ ] stay off [X] last state".
• Your PC's power unit is slightly faulty - probably a capacitor is a bit cooked. When it feels a sudden "spike" in voltage, such as you get when you switch an inductive load, it considers this as a temporary blackout. Therefore it resumes its previous state.
• Windows does not set correctly the "off" state (this is a common APM problem), i.e., the low level circuitry considers itself to still be on. Possibly this is because the desktop is partially on, e.g. to support wake-on-lan.
• when you turn the PC off from the button, it goes to "really off", so when it senses another blackout, it stays off.

You can break this cycle in several ways:

• use some spike suppressor (anywhere from USD 1,99 to USD 19,99) on the PC's power outlet. Note that they're power rated, i.e. if your PC absorbs 700W, you'd better use a suppressor rated at least 1kW, or it can either not work, or positively malfunction. This will probably stop the problem from occurring, whatever the real cause, by simply "hiding" the external spike from the PSU.
• disable "Power Resume" and/or "Wake on LAN" functions in the BIOS.
• there are several voodoo rituals that may make Windows convince the PC that it's lights out, really.
• replace the PSU. Shouldn't be needed, but this behaviour might be symptomatic of something worse coming. If you've got a slowly cooking capacitor, it may pop at some inconvenient time (they usually do).
• set the "In case of power failure" to "[x] STAY OFF!" if at all possible.