can i install a used 2.40 pent 4 processor in the place of a pent 4 1.6 processor
I see no reason why not so long as they're both 478 pin processors. BIOS might go into a bit of a flat spin on first firing of the new system but it should accept it. Check that Front Side Bus is the same in case you need to change the RAM as well.
Windows might then think it's been copied to another machine. If that happens, select the option to speak with a M$ adviser and explain that it's only a hardware change.