Leakage in case?

Hello, I have some questions. Today I decided to clean my desktop pc. Although I don't see very high temps, maybe 60C when I play a game and is hot outside(can't afford air conditioning anyway), summer is here so I thought I could clean up a bit the cpu, gpu and case fans. First of all, I'm not an expert at all and I also don't have the necessary equipment. I took some batonettes (you know the ones with which we clean our ears ) and I tried to remove some dust . Now my case has a metal plate at the base, behind and inside where the hard drivers are located. Now if I accidentally touch them with my arm, I feel a zap. Is that normal? There is also a spot at the base of the case in the corner, when I touch it with my hand I get a numb feeling. Is that ok? I read somewhere that when I deal with the pc hardware, I should leave the main plug plugged, the one with the 3 pins and close the PSU(there is a small button behind), also touch a metal surface while I do any work. Is this the right way? Is there a voltage leak or something like that? Thanks in advance.

EDIT: I should say that my PC works fine, no reboots or shutdowns, fast at desktop and games, I don't have any problem.. yet.

 

Thanks for the answer! How foolish of me to leave the power cord plugged :-o
I'll check into this can of compressed gas too.
I'm concerned about the charge, but as long as I don't have any problems I don't worry that much.

 

All so if dust does not come off easy with a can of air you can take a damp rag and clean each blade.Can of air does not all ways get the dust off the blades.Dust on the blades reduce the CFM or the Airflow.

 

Thanks, I'll keep that in mind. Although I'm mostly concerned about the leakage, do you know anything about that?

 

Well if your getting Zaped i think it might be static discharge.Sounds like the air is too dry.
Well what kinda leaking does it look like.

Does it look like Battery Acid.

Did you check to see if the quarter sized Battery is leaking.

Did you spill any kinda drink around the computer Water and computers don't mix.

Are you able to take a picture and upload it to here.

 

Damn it, I think I used the wrong word. Nothing spilled at all, I mean the zap I feel when I touch any metal surface, especially at the bottom. Sorry for my bad english. I don't think the air is dry though, where I leave the humidity is quite high.

 

Check to see if the screws are lose on the mob and check to see if any wire is touching the case like the P4 connectors.

Some times if the MOB is not tight enough on the case it could cause grounding issues.