DON'T leave the plug in the mains socket! This is extremely dangerous!
It has been pointed out before that a wiring error in your house can lead to the 'live' still connected to your PC. As a domestic socket only switches one lead, if that is the neutral you will have 'earth' and 'live' still connected. If you then earth yourself with the wriststrap...
The built-in resistor should protect you, but you will certainly feel something!
It is not neccessary for you to be earthed, just clamped to the same potential as your PC.
You may know more than me but do you definitely leave the power plugged into the wall socket (even though it is switched off).
According to Computer Weekly, an unamed computer magazine recommended this procedure recently (including attaching something conductive from the person doing the job to the pc case) and the Health and Safety Executive had to quickly send out a Press Release advising people not to do so as it could result in electocution!
I have always removed the power cord completely when working on the inerds of my machine.
Surely the computer metal case is earthed via the earth pin of the 3-pin mains plug. If this is removed from the wall socket you will lose the earth connection and clipping a strap to the case will be ineffective. I wait with baited breath to be corrected by the more knowledgeble members of the Forum.
You can get an earth strap for the wall outlet, ita a plastic pulg top ,the live and neutral pins are also plastic ,the earth is copper, there is a insulated wire from this which you can attach to the computer with a clip, has the same effect as leaving the mains pluged in to the computer but is a lot safer as only the earth is attached, lots of people just leave the power cable attached but as said above it is a bit dodgy.