I can encrypt my wallet file to protect it against theft.

But in order to perform a bitcoin transaction or to check one - or generally to work with the bitcoin client -
I have to unencrypt the wallet file. Right?

So where is the protection. A virus only has to wait some hours.
Or even better: My bitcoin client is always on.

Ok, I can split up the wallet.
One big wallet for saving and one smaller wallet (10 BTC?) for buying and selling things.

But again. At some time I have to put my big wallet online. A virus only has to wait.

What did I get wrong about your security concepts?