Internet won't allow me on some sites...

Hey, I have Telus internet at my house and most pages it will work fine, but some it will just say things like the site is down, etc. I normally use Firefox, and I checked on IE and it doesn't work either. I re-installed Firefox, re-setted my modem, even called the techies at Telus and they were clueless. But the weird part is is that when I use my neighbours unlocked internet, the site works fine. I do not have any firewalls set up or anything, and the site I go on worked fine on the my Telus internet a few days ago. Any suggestions?