My (almost) DIY built desktop PC started to give me electric shocks.

Hello experts!

My (almost) DIY built desktop PC started to give me electric shocks.

Here is the story of this computer:
- It was a P4, rather cheap bought from a store.
- It worked fine: Win XP, I play electric guitar plugged in and I played with it several months happily.
- I've found it too slow, so I bought a new motherboard, P4 too, but dual core and also faster CPU clock. I mounted it myself, was very careful of every point. I'm not a hardware wizard, nor a PC builder genius, but I followed very carefully the info from a website explaining everything in details with pics, etc. I also know some bunch about computers, but maybe I missed something.
- It worked fine for several weeks, if not months. I'm a guitar player and I played almost everyday with it. My system is as simple as possible: The electric guitar plugged in an effect box (Digitech Jimi Hendrix if you'd like to know), then the effect box plugged in the sound card. Either mic or line input, I have better resuts in lthe line entry of the sound card.
- Suddenly, I didn't change anything, but things are going wrong now: When I take the guitar, I got an electric shock! Quite strong, alternative, but not 220V. I have a tester and when I touch the strings, the light goes on.
- When I put the tester in my hi-fi amplifier chassis, it lights on too! And all metallic piece I test, the light is on: Equalizer, second computer (older PIII), modem, mixer, etc. Well, every chassis I test there is voltage on it.
- I unplugged the devices one after another, testing... plugging... testing... unplugging... testing... in order to find which device of my home studio is the culprit and finally found that it is my favorite (almost) DIY P4 computer, which I use everyday for everything.
- Yesterday, I could still test and see that there is some voltage on the chassis, but low because I was not shocked and thought maybe I fixed some problem somewhere without my knowledge...
- My bad. Tonight I played a little while on my guitar; No preoblem but after a short while I had to stop because I got shocked again.
- I don't know what to do, and would like some input from you on the possible solutions.

1) I'm thinking about cabling the PC's chassis on the current's earth. Maybe it is a mistake, I need expert electrical advice.

2) When testing the chassis, should I or should I not see the light on? I'm confused because I own a guitar amplifier which when I test shows current on the chassis: Never got any problems, even at night outside when it is humid.

3) I checked all 220V multi-plugs: The phase is correct: There is no inversion which could explain the shock: The problem is in this PC.

3) I'm sure the problem is with this PC. When I unplug it, and unplug every cable, every chassis of every device shows no current when tested. All is okay except this comp.

Thanks to help me. My last solution is to plug the guitar on a guitar amplifier, and record with a microphone, which will isolate my hands from the shocking chassis, but there will still be this dangerous computer I need to be fixed.