This isn't an electronic but merely an electrical problem.
Are the blades on your 2 wires the same width in that you can plug them in either way? Here in the US, one is wider to prevent the hot lead from being attached to anything you can touch and causing an electrical shock to your person. (Fire hazards too) If possible try the plug flipped the other way. What voltage is standard where you are? We are on 120V systems and the only other country I know is Britain with 220V.
Same question as I know not where you're from.
Are these screws connected to the case? If they are, current picks the path of least resistance and I'm surprised you can even feel any current by touching the case and the screws.
Do you have a voltmeter or VOM at your disposal? If the electrical supply in Taiwan is not grounded, I would make a devoted 3wire surge protected outlet(s) for my computers. The neccessary parts are easily found in any 'mart' and half the corner stores around here as well as electrical supply stores and Radio Shack's too.
You will need to find which lead is common with earth ground and which is not. The one with the wide blade is neutral AND is common with earth ground which is also your rounded ground lead. The narrow lead goes to the hot wire.
Now this problem could lie within your pc's as well and is not neccessarily a problem with your electrical supply. But I refuse to use my pc's in anything other than 3 wire grounded outlets.
What may be causing the slight voltages is sometimes referred to as a floating (or static) ground where the case is not connected to either of the leads and is sort of statically charged from things like transformers near enough to conduct a field current to your cases. Any coil can cause voltages to appear on nearby metal by 'field' current. Like how electromagnets work in a way if they weren't grounded you'd get lifted if you touched them and ground at the same time. In this case you could just run a ground wire from a screw on your case to a good ground like copper pipes that originate up through the basement or slab so they conduct to earth ground.
Lots of times the box that the wires are in are already connected to earth ground and you can just connect a 16 (14 is better) guage minimum wire to that instead. Voltmeters needed to do any of these fixes or measurements.
If any of you are having any unexplained fuzzy video lines, buzzing speakers or things of the sort on your pc's, it could be the problem being improper grounding.
My curiousity is definitely piqued, please post as you work this out.
PM is OK too.
There's no place like 127.0.0.1