The Trezor is safe from a potentially infected computer because the computer never sees the private keys when you sign and send a transaction. Here is how sending a transaction works:

You try to send the transaction from your computer, but the computer doesn't have the private keys, so you will be prompted to plug your Trezor into the computer. Once you have done this, the Trezor will ask if you want to send the transaction, and if you select yes, it will sign the transaction on the Trezor and then "hand" the signed transaction back to the computer, ready to be sent. The computer never has your private keys, and they are only stored on the Trezor itself.