Plugging in monitor causes floating ground on computer enclosure resulting in shock hazard
Firstly your computer cabinet is not earthed and it probably should be. Secondly your monitor's in-built switch mode power supply probably has conducted-emission-reduction noise capacitors from both it's AC line inputs to its earth wire (which may not be connected to earth either) but it may be connected to other "chassis" parts of the monitor which will also connect to other metal parts (on your PC) via the VGA lead.

This probably means that if your normal supply voltage is (say) 220VAC, due to the capacitors mentioned above, your unearthed metal cabinet may be being raised to about 110VAC due to the capacitors acting as a potential divider across live and neutral.