I have a computer which will not used, however, it is about 4-5 years old already,
is it worth to use it to mine BTC?
If yes, how much will I expect to get in 1 month
using your old computer to directly mine bitcoin is a hard thing to do. But you can still mining bitcoin but not directly to mine bitcoin but use your old computer to POS mining or staking for altcoin. Search for the profitable POS coin such as Diamond, Hyperstake etc, and then you sell your staking coin to bitcoinis it worth to use it to mine BTC?
If yes, how much will I expect to get in 1 month
It's not a hard thing to do. You can do it easily with a program or two.
But you will lose money with electricity on PC running. And not to mention you cannot compete against asics. 2 old usb block erupters can match sha power on hashing with great GPU's. SHA is a asic game now.