You may update your antivirus software religiously, immediately download all new Windows security patches, and refuse to click any e-mail links ostensibly sent by your bank, but even if your computer is running exactly the way it’s supposed to, a motivated attacker can still glean a shocking amount of private information from it. The time it takes to store data in memory, fluctuations in power consumption, even the sounds your computer makes can betray its secrets. MIT researchers centered at the Computer Science and Artificial Intelligence Lab’s Cryptography and Information Security Group (CIS) study such subtle security holes and how to close them.

There are always additional steps you can take like using a network-isolated machine for all the work and then moving pre-encrypted files with a thumb-drive to the network machines, etc., but it will always be a question of how much inconvenience you're willing to endure to ensure your data is secure versus how much inconvenience the interferiors are willing to endure to compromise your data security. As long as you're willing to endure more inconvenience then they are, your data will normally remain secure ...