Download and install ...

another browser such as FireFox, Opera, or Google Chrome and see if you can access the sites with the alternate browser.

If not, keep checking for Trojans using a scanner designed to detect trojans such as this online scanner - http://www.windowsecurity.com/trojanscan/ , or one or more of these -
http://www.emsisoft.com/en/software/free/
http://www.threatfire.com/
http://www.misec.net/papers/thvsbeast/
http://www.simplysup.com/