Computer won't power down

[ignotiev]

I work for a non-profit refurbishing computers for the poor. My latest project is an Emachines 550. Among other things, the power supply and processor fans won't turn off unless the unit is unplugged. Additionally, when the unit is powered down and plugged in, the light on the optical mouse flashes.

I postulated that there might be a problem with the power switch, so I disconnected it from the mobo and plugged the unit in. Same problems.

The power supply is a Ilssan ISP 120s and I can't figure out what the mobo is. Whatever they put in the 550.

Any ideas what could be causing this problem?

 

[ignotiev]

I unplugged the unit without grounding myself first, killing the entire thing. So I don't need any help after all.