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
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