Your company (and everyone elses) potentially has the ability to track EVERY website you visit (including the ads from sites. Some sites are ok, but they have prono ads), and every email you send and receive (and read them if they want), and every action you do on you computer (including potentialy seeing what you see on teh screen), look at and examine every file on your hdd etc. Basically everything you can do or know about your work computer (or laptop!!!) can be examined, read, watched by the IT guys.
There are MANY many people who have gotten fired for visiting inappropriate sites, and its not just pron either. People have also gotten fired for blogging about work. Sometimes its enough to visit sites that the boss doesn't agree with politically. So ask your IT guy whats appropriate. Like for instance it might be ok to check news during your lunch hour, but not other wise. Also examine your written policies to see what they permit and prohibit. If you like fark, make sure not to visit any sites labeled NSFW (not safe for work).
It would REALLY suck to get fired for visiting some news sites, so make sure you are proactive and find out what is allowed and what is not. Written and unwritten. Like the computer policy may have a blanket statement about visiting sites on work time (or even during lunch hour) Or just dont use the internet at all for personal use.