I had a similar thing happen about 4-5 yrs again, with a P4 system I had. I brushed my arm against the case one day, and felt a tingle.
Which, I knew didnt sound right. What happened in my case, was not only was the power earthing the case, it was also frying the CPU (the system kept crashing), and it was screwing up the videocard. If I turned it off and disconnected the video cable, and touched the pins, they were live
I found out what the prob was after. The earth wire had come out of the power point, it was plugged into. Lucky for me, I got onto Intel, who replaced the CPU for free (the fried CPU had to go back to Malaysia to get replaced). I used a lower spec CPU, in the meantime. The replacement took 2 weeks to come back. I had to replace the mobo tho. After that, it was all good