My PC (an old pentium2 333MHz) makes a strange noise as it boots and doesn't accept any keyboard. The keyboard is OK. The m/b takes a DIN keyboard and i use an ps2->DIN adaptor, because my keyboard is ps2. I also tried a serial->ps2 adaptor,in order to connect my keyboard on the serial port, but i have exactly the same problem.
What do you suggest?

Could you elaborate on the strange noise? Does it make the same noise without a keyboard attached? Is anything displayed on screen?

How do you know that the keyboard is fine?

Have you tried another keyboard? Preferablly one with a DIN connector. . .

You're not going to be able to connect a keyboard to the serial port of any PC. On higher-end mini-computer/workstations/etc usually found running some flavor of a Unix System V decendent you can often attach a terminal (keyboard + CRT) to the serial port or another PC and access it via a serial communications app.

Take the cover off the machine and find that noise! The machine might be seriously broken in some other way.