After some strenuous days, let me report back: 1..From win-7, I manage to install win-8.1 on another partition. But with this type of installation (without boot), was impossible to reach BIOS via win-8.1. 2..During the second installation I erased the two partitions on which win-7 and win 8.1 were installed 3..Quit and Restart 4..Somehow the BIOS get the information that, there is no operating system and mercifully allow me to boot (pres any key to boot...), to chose from where to boot (F12) and finally to enter BIOS (F2/DEL).