Blocked from some websites

I was able to go to any website I wanted, all the sudden one day I was not able to go to some (very few) websites including Hewlett Packard’s website.
Any time I want to go to their site my browser looks as if is going there, but it just hangs until it says “Page not found”.
It doesn’t matter if I use Explorer or Netscape the result is the same.
I have to use another computer in order to go to those websites.
Does my computer has a cookie or something that impedis me from going to these websites?