Despite being a card carrying Labour Party member and activist I am also an employer owning a small (very small) web design company. As it is in the interest of the company for all staff to be familiar with the technology we encourage private use of the internet during work hours. But our circumstances are an exception.Our main priority is that the work gets done properly and on time. If that is achieved we don't care how much "free use" is made of our web connection. If work ain't up to scratch because of time wasting then we have to kick ass. But generally, our trust in our staff is rewarded with respect and honest work.
While I frown on any employer secretly monitoring the private actions of employees wjile in the work place, I can see that larger organisations than ours would descend into chaos if run in the liberal way we operate our little enterprise. Fair and clearly understood controls must exist.
When in doubt, expect your work PC to be monitored and checked. If you want to use your PC for private use, say it to your boss and ask for permission. Email the request so that you have a record of the request and reply. In most cases, if you play fair with people they will play fair with you.
Frank