This is due to a bios problem, take the bios battery out and then clear jumpers at the motherboard jumpers to set system configuration options. Jumpers with more than one pin are numbered. When setting the jumpers, ensure that the jumper caps are placed on the correct pins. Also do a bios update (if your bios is old) but be careful as an unsuccessful and wrongly done bios flash will brick your desktop.

A single, constant beep at power up is an indication that the memory isn't working. What memory do you have installed? We don't have references that take a motherboard number and tell us what computer it is out of, hence the request for your model number. Different computers will likely have much different specs.

@RMO_emanuele it is best to start a new thread for your question. The original poster here never gave us the model number of their machine, so we weren't able to answer their question fully. The front panel connector is usually the same as other designs, but if you give us your full model number we should be able to tell for sure. Your full model number is usually on the same sticker as your serial number, it should look something like xxxxx-xxx-xxxx.