Anyone has an idea how to remove this ad(virus?) from my small laptop screen ?

Do you use AT&T for internet access or anything on your computer?  First thing I would check is under Add/Remove programs on XP and Uninstall a Program under Win7 and look for anything pertaining to AT&T and if you don't know what it is or use it, remove it. Then you can go download malwarebytes and do a scan to see if anything shows up.

thanks for your reply. it is not relation with AT&T application. the ad changes about 1-3 times every minute.

it is only visible when i am using google web browser. i guess it bind with the web browser

Try one of this tools
http://www.kaspersky.com/virus-removal-tools
http://www.kaspersky.com/antivirus-removal-tool?form=1

Or (my recomendation) make a full scan with this tool with kasperky Rescue Disk
https://support.kaspersky.com/4131

Check your Add-ons if you are in Internet Explorer.

Go to Tools (if you can't see it, hit the ALT button), Manage Add-ons.

Disable any Non-Microsoft Toolbars and Extensions that you don't recognize, then close out of IE and reopen.

SSharma is right. i am using MalwareBytes and already removed ad at bottom of my screen, ( first run it in quick mode, and then run in full mode), but i cannot remove ad on the right side of my screen. i dont care about it too much because my screen width is very big. if you know how to get rid of the right' ad, that would be great.

that's fantastic. thank all of you.