As has been stated, do not use your work computer for anything other than work. I do disagree with Dood to a certain extent. Some companies will allow you to go to sites, simply so that they can track policy violations and potentially use this data against you should the need arise. It is also not so much the IT department reading your stuff, agreed no time, however, here in Vancouver I have installed many different types of surf tracking, keystroke tracking, monitoring, proxies and video tracking software and hardware allowing my clients to track and monitor the usage of their computers. They can choose to monitor specific employees, groups or all staff. Other than blocking sites, they have chosen to have employees sign Internet usage policies and then monitor for violations. It is very easy to parse a log looking for keywords, sites visited, and then tracking such words to a specific workstation, date and time of day. They are not interested in reading your email, just interested in how much time you waste going to various email sites, social networking sites. any sites not considered politically correct (Porn and PERB) or any site that could potentially damage the Network.
As stated by others and myself, you have no expectations of any privacy on a company computer or while attached to a company Nertwork, so do not expect any and get yourself a smartphone, unless it is a company smart phone connected to the company WiFi.
JMHO
D