As companies deal more and more globally, it's easier and easier to have information from different regions, each with its own rules. The E.U.'s Data Protection Directive, for example, permits individuals to access computers that have information about them and requires the holder of that data to modify it as requested. Canada and Japan have similar laws relating to personal data.
In the United States, the general rule is that employees are not entitled to privacy for emails accessed through email systems provided by the employer. On June 17, the U.S. Supreme Court voted 9-0 that employees should likewise not expect any privacy for text messages accessed using employer-provided equipment. However, employees can expect their emails and text messages to remain private if accessed only on their personal equipment. An employee using a personal iPhone or PC for work email could expect personal emails on that device to be private but not emails accessed from the corporate email system; many courts have ruled in the United States that the use of corporate email systems mean that the employee should expect no privacy.
Another area in which you should be careful: You should not access confidential information for personal use. That sounds obvious, but some courts may think that reviewing confidential information is not an innocent activity and assume there's an intent for personal use. You should have a specific business reason to review such information. On the other hand, one federal appeals court overturned the conviction of an IRS employee for reviewing taxpayer information inappropriately because that employee did not actually use the information.
Pornography use In the United States, possession of child pornography is a crime, and there are no ifs, ands, or buts for this issue. If you find child pornography on computers at work, you could also be guilty of a crime if you do not turn in the person possessing the child pornography. This fact puts a big burden on each IT professional to be vigilant for child pornography. But note that the U.S. Supreme Court decided that an animated video featuring cartoon characters of kids was not child pornography because there were only computer-generated characters.