Wired Magazine does an article about Vista. here is the relevant part about Internet Filtering.
http://www.wired.com/news/culture/reviews/0,72295-1.html?tw=wn_story_page_next1
Parental Controls I turned on Vista's parental controls and selected them for my daughter's account. Access to certain websites -- as well as games with a mature rating -- could be blocked. When I tested this feature using my daughter's account, I found that English-language porn sites could not be accessed.
One flaw I found is that the website-blocking feature is not worth much in a multilingual home or office setting. Good ol' American porn sites were blocked, but I had carte blanche access to the raunchiest of raunchy French and Spanish sites. I was also able to use Google to search for vulgarities in those languages.
I found this particularly aggravating since I live and work in France. Microsoft is able to detect my France-based IP address, and I know this because it imposes French-language web pages on me when I try to access its help sites. If Microsoft can figure out how to switch over to a French site based on my IP address, why can't it make its website-blocking feature multilingual?
Admin privileges also allow you to track the websites your kids visit and e-mails they send, among other user activities. Whether you choose to do this or not is up to you.