It's my guess that you have recently downloaded some software of your choice and Blockandsurf has, unknown to you, been included as part of a package. It's the sort of stuff that comes along with programmes for recording on-line videos that are often subjected to copyright. I am Not suggesting that is the case so far as you are concerned because there are lots of reputable sites which also include lots of Pups (Potentially unwanted programmes.)
However, before following all that is suggested in the excellent link which Graham has given, I would be inclined to give a scan with Malwarebytes to see what can be established as nasties/naughties. The reason I say this is because re-setting your Browser could cause you further problems. Incidentally,what Browser/s do you have and what operating system are you using (Windows Vista; W7 or whatever).?
Hans.