There is some insidious malware around that never used to be.
If the above tried and true suggestions fail to fix your problem, then go here
Tech Support ... and read the "Security" sub-forum. Have a read of some of the posts there or do a search.
A malware called Cool Web Search has been doing the rounds lately and (apparently) can only be removed using a specific tool called Cool Web Shredder. The malware is continually being changed to avoid the fixes for it, so the CWS remover is constantly being updated. All the info is in the above link.