I reconnected the 'puter to earth and disconnected the audio monitors, it's sounds great, no noise at all..
For now I'll lett it this way untill I find something better or find a different solution... I've talked to another guy from a music store and he told me that what I did is what they advise the clients to do in a case like this...
... My brother sais the same and does it for ages, nothing happened 'till now... One thing is sure I can't play with all that noise.