The initial responses seem to be alluding to wallet-stealing malware.

The thing to remember is that proof of ownership with Bitcoin is knowledge of a private key. For large amounts, the private key should not even be stored on an Internet-connected computer.

Because transactions are irreversible, computer security suddenly has money on the line.

I recommend paper wallets. Of course, you have to keep in mind that if anybody is able to read the keys without your knowledge, they would be able to spend the coins at any time.

I also recommend keeping more than one copy in at least 2 geographically separate locations. While this will increase the risk of theft, it will also reduce the chances of loosing your Bitcoin in some disaster that destroys one location.