Encrypt your file system, don't be a terrorist.
How does one do this?
It depends on the operating system and the file systems being used. I use Linux and Ext3 or later mostly. I encrypt certain files on my desktop computer and my file system on my laptop. It is not something I'd want to explain because then people would hold me responsible for tech support. If one knows their file system (and OS), it can be researched online easily. If one does not know...then this is the least of one's concerns.
This would prevent unauthorised snooping. It would not prevent social engineering or compensate for poor security in other areas (anyone here who shares a computer account with anyone is a security nightmare and should get the basics down before taking such measures).
Also, one can be legally compelled (in that it would be a crime to refuse) to decrypt a file system in certain situations, however, if one is in this position, one is already arrested and charged with a crime. This sort of action is usually used against suspicion of images of child abuse that were glimpsed by reliable witnesses already and when ownership of the computer is already admitted. For other things, there can be legal recourse because if producing documents or files would be incriminating by itself, it would be illegal to compel one to do. On the other hand, some laws are applied to computers when it may be inappropriate. Technology hasn't really been treated maturely by the legal system.
United States v. Boucher and United States v. Hubbell are my sources (inappropriate cited I know, but I do not have access to proper reporters at this time). Just be careful not to be in a special situation, like dealing with the military or crossing a border. The rules change when then.