What is this private key you speak of?
Make a backup of your wallet, then make another backup onto a different medium. Then put the backups somewhere safe.
There is a wiki post on security - read and understand.
('Wallet' is a misnomer. The coins are never 'in your wallet'. The record of all the coins and all the transactions are in the block-chain, the wallet only holds your keys, so that it can pick out your transactions. If someone gets hold of your wallet or private keys, then they get control of any coins associated with that key, even transactions in the future. If you think your wallet has been compromised, get a new one and never, ever use the old one again.)