Your boss almost certainly as the ability, and the legal right, to monitor your email and instant messaging conversations.  You should assume that any email you send on a company-provided computer is being stored and may later be reviewed by your employer.

In a new video titled Is Your Boss Snooping?, ABC News points out that the best way to find out if your boss is reading your email is to just ask.  Most companies will be open about their policy.

The StealthSurfer comes with a subscription to Hushmail (the encrypted email service) and Tor, a popular program for anonymous web browsing.  These tools can help protect you against snooping by your employer and even by hackers.