Help!Cannot remove a virus

My computer has been infected with an unknown virus.Format doesn't help, Avast!,Avg, Bit Defender,G-data cannot remove this virus, because of it my computers performace gets to 100%(i hawe a 3.06ghz dual processor and 1gb ram) sometimes a windows.dll loads and then the performance gets even worse i cannot connect to internet,and the blue screens of death are killing me.i dont know what to do, im thinking of buying a new hdd but im affraid that it wont help what should i do???