If you are behind a serious firewall or in front of one for that matter, the filters will block by content as well as Domain Name, IP etc etc.
In short this rarely works cept at small universities. But then if they suck that much you could just use the shite 'at' cmd hack and hack the LAN to give you all the net user privileges you want. But most XP systems when in user group won't let you use the 'at' cmd.
Last but not least is Torpark, a firefox TOR combo, that you can stick on a thumb drive.
I've spent way too much time trying to get past my company servers and they finally got serious and stopped just blocking IP and even Obfuscated addresses, they blocked marked content. You can look at a site once and it will be fine but then if you try and open the URL again its blocked.
However, TOR + Privoxy + Foxy Proxy and FrieFox solves most of those problems. The only porblem with any solution you can find is that its slow as a mofo.
Oh and there is one HUGE hole in all XP systems, if you boot from Win2K disk to the repair console you can have full access to all the NTFS files without hacing to know the admin password.
Now I'm babbling. Sorry, been a rough xmas.