If I were you, I would use a hexagon spanner, whether metric or imperial I can't say.

The power supply unit could no longer supply my computer with power, there probably wasn't that much voltage left ^^
And no, capacitors in power supplies are too small to store life-threatening voltage for a long time. I'm referring to the time span, which is why I only used tools with plastic handles.

Never open a power supply unit. Power supply units have capacitors that can store life-threatening voltages for a long time.