What is your antivirus? Which browser you are using? Internet explorer/ Firefox?
(1)Disable
anti-virus software temporarily and check:
(2)Try add-on free mode..
Press Windows key and R Key and type
iexplore.exe -extoff
and click ok...

(3)Reset
Internet Explorer settings and check.U
will loose your favorites/bookmarks and thirdparty plugins by reverting default settings ... if you need you can backup..
http://support.microsoft.com/kb/923737/en-us
(4)Try
a Clean Boot to check if there is any third party software conflicts. (dont forget to reset
the computer to start as normal after troubleshooting)
http://support.microsoft.com/kb/929135
(5)May be its due to this malware "ZeroAccess
rootkit/Sirefef"... What it does is it will block downloading and it will block you from visiting to some site...
Download and transfer this....
Use one scanner at a time
Download hitman pro 3.7 from uninfected pc and transfer to you pc and install (shareware and can be activated for 30 days free)and have a
full system scan.. http://www.surfright.nl/en
alternatively you can use hitmanprokickstart usb bootable
How to create a defender offline bootable cd/pendrive