My computer is blocking sites and i don't know why!?

My computer isn't letting me sign on to facebook, or yahoo [i'm on a friend's computer] and I don't know why. I've been on the security settings and set them both to trusted... but it's not working. What do I do?!

My computer is blocking sites and i don't know why!?malware

maybe its the pop-up blocker turn it off or change it/un-install it

My computer is blocking sites and i don't know why!?norton 360

Check your security settings. Too high of a level will cause this to not let you sign in.

Unless you are on a wireless router i can see any other problem unless there is software loaded on that machine to block sites.
pop up blockers may be one issue, but when you allow sites you can only allow them in one area only. to over ride any action all you have to do is hold down ctrl and O for over ride..you may also need to import certificates for the sites so they can be properly recognized on your PC
Kill the Firewall program, or try Mozilla Firefox, it works WAAAAAAY fater then IEXPLORER.
What browser are you using? Try another one....

And can your internet itself just be down?
Could be Parental controls are enabled.
It could be possible that an outside software or setting (like parent controls) are preventing you from signing on. If anything, someone could be blocking you from the site OR that computer is ISP banned from that site.