They can tell all of your sites visited by looking in your history files and temporary files. Even if you do delete the history... they can look at it. But usually they won't go through this much trouble... unless you're labeled a threat or something of that nature. They can't read your emails from a personal site... but they can tell that you visited the site.
They might have software installed to see what sites you go to but its unlikely they can see what you do for example. There are programs such as key loggers that record every keystroke you make thus allowing them to see exactly what you do. Its not likely they have such a program on a company PC. As with all things regarding the internet if you dont want people to know what you are doing, dont surf/email/ etc on their computers. Its a risk for sure