My PC fail to turn ON

MY PC (Intel pIII 1 G Hz with 64 MB RAM) was alright and worked at its peak performance. When I have opened its cover for PCI card( internal modem) installation, I found it was dusted heavily. I cleaned it using soft brush including motherboard. I restarted my PC without installing PCI card, but it fails to ON. NO sound, no led, no fan rotation, nothing…machine is dead. I replaced my PC power supply (SMPS - ATX) with another good one. But no use. THAT IS my PC POWERSUPPLY IS ok. Even I disconnected all drive connecters, just connect 20 pin power supply on mother board and checked, but no use. I measured voltage at 9 no pin of 20 pin connecter from its top without removing it from the mother board. It should come 5 V DC, even without pressing PC ON button from the front of PC. Yes it is coming. Then I traced the two pins on Mother board, where PC ON push button switch wires connected through its 2 pin female plug. One is ground. Another should carry voltage of 5 V . Instead it comes about 4 V. What it should actually comes, I do not know. Is this low voltage (4 v) caused my PC dead? I also checked PC ON push button switch. It is OK. No doubt on this. Please help me.

The 5v supply has some margin for differences but I think it is only something like 4.7v to 5.3v. It does seem to me that 4v is very low.