using either the Console in GUI, or the bitcoin-cli... You can use the dumpwallet or dumpprivkey commands (If you have a passphrase for the wallet, you'll probably need to unlock the wallet first)
walletpassphrase "passphrase" timeout
dumpwallet "filename"
dumpprivkey "address"
"Passphrase" is pretty obvious
![Wink](https://bitcointalk.org/Smileys/default/wink.gif)
The timeout is the number of seconds to unlock the wallet for (ie. 600 = 10 minutes)
"filename" is the filename to dump the ENTIRE wallet to
"Address" is the specific bitcoin address you want to dump the private key for
examples:
walletpassphrase Th1s1sMy5uper5ecretPassPhrase 600
dumpwallet c:\dumped_wallet.txt
walletlock
walletpassphrase Th1s1sMy5uper5ecretPassPhrase 600
dumpprivkey 1BvBMSEYstWetqTFn5Au4m4GFg7xJaNVN2
walletlock