How to remove popup box inside browser

[pcwin]

I keep getting a popup at all my browsers (firefox, internet explorer, chrome) at random times.

The popup displayed this message: sin1.g.adnxs.com say: You won an iphone (screenshot http://i.imgur.com/at344LM.png )
I have clicked x but it still popups after some time.
I have clicked cancel but it still popups after some time.

I have followed these steps at Remove ib.Adnxs.com pop-up virus (Removal Guide) but the popup still happens.

How can I remove the popup?

I have spent lots of time but I still could not get rid of the popup.

Thanks a lot.

 

[Ztruker]

Did you follow all the steps at the remove ib.Adnxs site?

STEP 1: Uninstall the adware program responsible for the ib.Adnxs.com pop-ups
STEP 2: Remove ib.Adnxs.com adware from Internet Explorer, Firefox and Google Chrome with AdwCleaner
STEP 3: Remove ib.Adnxs.com pop-up virus with Malwarebytes Anti-Malware Free
STEP 4: Double-check for the “ib.Adnxs.com” malware with HitmanPro
(Optional) STEP 5: Remove ib.Adnxs.com pop-up ads from Internet Explorer, Firefox and Google Chrome​

The list looks pretty comprehensive to me and should do the job.