What is this and why it is not going away offers4u.org?

Last few days offer4u.org adware has been bugging me. It keeps popping up every browser (explorer, fire fox, chrome). Have tried to uninstall it but keeps coming back. 

Have tried

1.   Full System scan

2.  Clear temp files

3.  Computer registry search to find it

4.  File search

Nothing is working, please help. Thanks