Try each step one by one.
Any one step may work for you. Example – If step 2 solves your problem then you don’t need to continue to further steps.

1. Go to Tools – Options in your browser and then delete all browsing history and cookies.

2. Restart your PC and your Router.

3. Go to Start – All Programs – Accessories – Command Prompt – then type “ipconfig/flushdns” (without the quotes).

4. If you use Google Chrome or have the Google Toolbar installed – Go to the top right corner and click on the “Adjust Google Toolbar Options”. Then go to options and uncheck “provide suggestions for navigation errors”.

5. Go to Start – Control Panel – Network Options – Right click on your network icon – Go to tcp/IP and click on “Automatically Detect Settings/Obtain IP address automatically” – save and close.

6. (If you have already set to automatically detect settings and do not need step 5).
Go to Start – Control Panel – Network Options – Then click on repair connection

7. Uninstall your browser and Install it again.

8. Update your anti-virus and then scan your computer.

9. Remove your temporary files. Here’s a nice little program which can quickly remove all temporary files.
ATF Cleaner

10. Download Malwarebytes Anti-Malware
After installing it, click on the Update Malwarebytes Anti-Malware and then launch the program.
Perform a quick scan by selecting all your drives and then remove any malware if found.
(If you cannot access anti-virus sites because of the problem, download the program from another PC and then copy it to your PC using a flash drive or an external Hard Disk.)

If none of these work, you can go to this forum which has friendly moderators who will personally help you once you post a thread with your problem.
Geekstogo

And if nothing works, you can always format your PC. Formatting your hard drive is actually the best solution especially if you use Credit Cards to buy online, do online banking transactions or have confidential information in your computer. Backup all important data and disconnect from the internet before reformatting.

To find out more about reformatting your computer check this : Reformat your PC

Good Luck!