Even if you use a file-shredding program consistently, law enforcement officials can always use a variety of computer forensics tools to pry out any secrets your deleted files may be hiding. So how can you protect your computer from their prying eyes? Basically, you can't. While you can make recovering data harder by periodically purging your cache directory and only storing files on removable disks (such as floppy or ZIP disks) and physically destroying them afterwards, just remember that everything you do on your computer can be recovered and examined later.
Even if you use encryption, guess what? Any information that you encrypt could still be stored in slack space or temporary files, which means a forensics expert can avoid your encryption and uncover your information by finding the unencrypted version of your data elsewhere on your disks.
If you don't want your computer to incriminate you, learn what computer forensics experts are capable of recovering. Experiment with some of the free or commercial forensics tools to recover data on your computer, or see what you can find on other people's computers. Try secretly examining a coworker's computer. You might learn how to better protect your own data.