If there is a virus on your computer, and your bitcoins are stored on that computer, then there is nothing the bitcoin software can do to prevent that virus from eventually stealing your coins.
That said, allowing you to 'lock' your coins with a password, and requiring that you enter that password to send coins, is high on the list of things I'd like to see bitcoin do. That would make it harder for a virus to steal your coins.
But even then, a smart virus could lay in wait until you typed your password to unlock your wallet and take that opportunity to either capture your password or send the coins to a bad guy. If you can't trust your computer, don't store your life savings on it (and yes, bitcoin software also should make it easy to save some of your bitcoins on USB sticks or CD-R disks so they can be stored safely in your safe deposit box at your bank).
That said, allowing you to 'lock' your coins with a password, and requiring that you enter that password to send coins, is high on the list of things I'd like to see bitcoin do. That would make it harder for a virus to steal your coins.
But even then, a smart virus could lay in wait until you typed your password to unlock your wallet and take that opportunity to either capture your password or send the coins to a bad guy. If you can't trust your computer, don't store your life savings on it (and yes, bitcoin software also should make it easy to save some of your bitcoins on USB sticks or CD-R disks so they can be stored safely in your safe deposit box at your bank).
As I've mentioned in other threads - a bitcoin wallet isn't that big - you could easily print it out with a high quality printer and some fancy software, then store it in a bank.
I don't see how printing your wallet.dat would help to protect you from theft of your coins.
This was assuming you're holding onto the wallet.dat as a long term value storage option - print out the wallet, mark down the addresses, and delete the original file. Then store someplace safe
Easy to deposit coins to the wallet, but hard to withdraw.