help please, wireless not working

hi, the internet on the main computer in my house isn't working. it says its connected but using multiple browsers it can only open one site( the router page so it obviously is communicating with the router) and i'm currently sending this message on another computer on the same network with out problems. anyone have any ideas what could be causing the problem. the only change that was made to the computer since it worked was that the dell tuneup was installed and used but after trying to search for anyone with similar problems i couldn't find anything. anyone have any ideas out there please, any help is greatly appreciated( spent 5 hours trying to fix it yesterday)