Problem
Service Pack 3 installation interrupted before it’s completion afterwards the computer is restarted.

Data
System Event viewer shows a problem loading nForce Bus device. This points out to the network bus driver which enables the Ethernet controller.
Latest driver version of the network driver Ethernet driver 4.75 (WHQL). Latest available network driver Ethernet driver 4.75 (WHQL).

Solution
Boot in Safe Mode.
Install Service Pack 3 on nForce4 chipset.
Reboot.
Log in Safe Mode.
Unistal the nForce Bus driver.
Disable the device.
Reboot.
Reinstall the Network Bus Driver.

Conclusion
I have stumbled upon three such cases, before finding an adequate solution of the problem, but I did not test any other combinations such as reinstalling the computer with an integrated SP3 installation and so on. Anyway the above procedure solved my troubles.