Well actually perhaps I should add that it is quite a remarkable shock not just a little tickle. ALso I didnt mention that in my house I do not have ground so the PSU isnt discharging. I do however have other computers and I dont have the same problem with them!? :?
I had this problem with a computer once. Something is wrong with your build to be feeding a noticible amount of electricity back through the ground. Also, it might be plugged in backwards (110v is supposed to be AC current on one side, and "neutral" on the other side).
The quick fix: take that 3prong to 2prong adapter you're using. You see the little tab for the ground? GROUND IT (that's what it's there for). The scrwe in the middle of the faceplate of the outlet *should* be grounded, but it ussually isn't. Wrap a fairly beefy wire (I used speaker wire) around the tab so it is secure and will conduct electricity well, attach the other end to a ground. What you use for a ground can be just about anything, but it's best to connect it to the same ground as everything else in the house so you don't get votage flowing back across it.
Choices are:
1. The negive side of the two-prong outlet (it goes back to the exact same place a "ground" wire would anyway, just make sure you attach it to the negetive side, not the positive side). Pros: eaiest, best solution from an electrical EMI standpoint. Cons: riskiest, if you hook it up backwards you will fry your computer and possible start a fire. Frequently these old two-prong outlets are wired BACKWARDS, so if you unplug it from one outlet in your house, and plug it into another... smoke.
2. Copper water pipes. These are grounded. Pros: works well if accessible. Cons: Prone to lightning damage or could possible electricute someone to death while taking a shower (that's why they don't do it anymore) but your wiring is problably grounded there anyway so it's not making anything worse then it already is.
3. Central AC ventalation, baseboard heaters, steam heaters. These are ussually grounded. Pros: works well if accessible and grounded. Cons: none.
4. Run the wire outside and wire it to your grounding spike (if you have one, you probably don't, your entire house is probably grounded to your water pipes) or make a new grounding spike (on the cheap: dig a hole, strip a bunch of the wire and put it down there, fill the hole back in).
To fix your PC:
1. take the PSU out, plug the PSU in, see if it electricutes you. If so, get a new PSU. If not go to step 2
2. Unplug all drives and cards and check all mounts on the motherboard to make sure nothing is being shorted out (check especially under the motherboard). File down offending parts of the motherboard tray, use plastic and those little paper washer thingies to make sure your mobo isn't touching anything. Put the PSU back in and power the system up (yes, with no GFX card or anything. It won't boot but we that's ok) If it still electricutes you get a new mobo, if not go to step 3. If the mobo tray is all acrylic this really should be the problem though. Just check the screws.
3. Start plugging cards and drives back in one at a time. The one that electricutes you should probably be thrown away. Make sure that it's plugged in properly and not shorting anything out.
Once you think you've isolated the problem to either the PSU, mobo, or ond of the components try swapping it out and see if the problem goes away.
Make sure you follow proper ESD and handling proceedures, especially unplug the PSU from the wall inbetween each time you add or remove a component.
Once you ground that tab the problem will basically go away. The Acrylic case isn't causing the problem, it just isn't helping it go away either. You really should try to figure out what is causing it and fix that then ALSO ground the tab.