Is it possible that it may be overwriten because before I manage to recover it I've been able to open it in text file and it was readable wallet.old
It's possible, but it depends on how the HDD is used. If the HDD used to store heavy stuff such as OS, 3D application/game or write lots of files, it's very likely the wallet file is overwritten.
However, if you're serious to recover your wallet, first thing you should do is make RAW copy of your HDD.
exactly... Shut down your system right away, remove the HDD from your system and use a tool like dd to clone your disk... The longer you keep using the disk, the less chance you'll have recovering anything. Maybe you won't need the clone, but it's better to be safe than sorry!