Frequently Visited Websites Won't Load?

Some websites won't load for me, and I mean Websites that I visit frequently, an example being Youtube. And I know the sites aren't down, as I saw them load on my other computer.

Also, I've tried lots of things, Such as Virus Scan, Spyware Scan, Turned off Firewall, and I just can't do it.

BTW, I'm using Firefox, but that doesn't help, as I used a different browser (Opera), and I have the same problem.

Can someone please help me?

and, here's some extra details:

I'm using Windows Vista 36-bit, on a Laptop, and when I try to load the pages, the computer says this: Firefox can't find the server at www.youtube.com, like the website doesn't exist. And I'm the only person who uses this internet. So really, this is annoying me.

And I'm also using Windows Firewall, and I've turned it off so I could access the webpages, but it had no difference.

Some other websites work, while some sometimes work.

I hope I've supplied enough information for you to help me.

I just had this problem over the weekend, and we spent a lot of time trying to troubleshoot it. What a headache it's been! We did everything that you did, and we even did a fresh install of our OS.

The solution we found was to go into our router's WAN settings and change the MTU from 1500 to a lesser number. We set it at 1400, saved, closed the browsers, re-opened them and I am now able to load the pages again.

I hope this will be the fix for you.