How many pixels are there in one millimeter (length and not area) of a standard PC screen (I don't know what is the standard size of PC screens):
a- When using a 1024x768 screen resolution.b- When using a 800x600 screen resolution.
It'll be different on each monitor - 14', 15', 17', 19' and bigger.So there's no one and universal value for this.
Out of curiosity, why do you need to know this?
Pixels are not a unit of physical measurement, so any comparison you try to make between pixels and millimeters/inches/miles will be flawed and inaccurate from the start.
And as a sidenote, this is why 72 pixels/inch as a »standard screen resolution« really doesn't say anything.