how can i block a given webside by usin avast

Every day i am getting a webside i do not ask…I can only solve the problem by restarting my computer. Can avast help me to block this annoying webside

That sounds like a infection, maybe a browser hijacker.
Please follow the instructions and ATTACH the logs to your next post
http://forum.avast.com/index.php?topic=53253.0