Blocking websites
 

Upon discovering something, I have found that I need to block access to specific websites for certain users on my computer. Now, I could get some internet filtering software to block whatever I felt like for certain users, but I'd like to avoid doing that.
The problem comes in that I don't want the websites in question to be blocked for all users on the computer (so a HOSTS entry pointing to, for example, 127.0.0.1 won't do the job), and blocking within IE won't work because Firefox is the only browser used on this machine.
My question is, therefore, is there any Firefox plugin or other method that doesn't involve internet filtering software, that these sites could be blocked from specific users? (Although, if nothing can be found, I'll just use some sort of filtering software, in which case recommendations are requested. Also, yes, this is on Windows.)

Do all persons on your computer use a different XP session?

If so, it would be pretty easy to arrange something with Firefox or Peerguardian.

Yes, they do use a different session. (This is Server 2003, though.)
The one suggestion I got (over MSN) was Adblock, which seems to work well. I had suspected Peerguardian as well, but I wasn't sure if its settings were per-user.
I will give Peerguardian a shot, especially if it--like it should, via various hiding options--allows for a more transparent blocking method. (That is, with Adblock, I can't find a very simple way to remove all traces of Adblock from Firefox.)

Well, it all depends on how experienced the persons you want to block are. I know my close relatives wouldnt be hard to trick as they don't know that much about computers, but heh.

That's the case here; I just hope that the one I'm most interested in disallowing access doesn't decide to go exploring one day. Thankfully, group policy prevents most possible exploring.

Thanks for the help, though =)