If you can access most sites, but only a few can't be accessed, it's probably not caused by any addon. Removing your antivirus is a bad idea, even when diagnosing most problems. If not done already, reinstall your antivirus.
Open the hosts files. It's an extentionless plain text file. It's located in \Windows\system32\drivers\etc\
You can open it with notepad.
(on a 64 bits OS, it may be \windows\system64\drivers\etc\)
Do you find any of the problem sites there preceded by 127.0.0.1? It's the local loop back address. It's effectively a "do nothing" address.
If so, just delete those lines and save the file.
Maybe some body played a prank on you...
Are you using a router between the cable modem and your computer?
Most offer web sites filtering. In some cases, it can be set on a per device basis. It's a feature that can be used to block any access to some sites.
The exact location can vary. Look around.
If you see any entry with the name of the site, or even a part of it, erase that entry. Save the change.
It could have been a firewall configuration isue, but removing it did nothing, so it was not the isue. Reinstall your firewall.
Electro
October 2013
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix notebooks/laptop pcs.