The answer is simple. The company who owns the computer has the right to monitor all business conducted on that computer wheather it is business related or not. Most employees think that the pc belongs to them and the have the right to do with it as they please. The pc is there as a tool for you to accomplish your job. If you conduct personal business on that pc then you will be monitor. They are most likely doing this to protect company secrets and also determine who is browsing the internet instead of working. They should have a policy letter or a disclaimer to let you as an employee know that your pc usage is being monitored. However it is not mandatory that there be something in writing. There are all sorts of programs on the market that will track pc usage.