Any error messages onscreen?
Have you checked Event Viewer for any errors...which might illuminate what could be impacting your connectivity?
How To Use Event Viewer - http://www.bleepingcomputer.com/forums/t/40108/how-to-use-event-viewer/
My wild guess would be that something...that loads with normal boot...is preventing your Internet access.
No access at all...or specific sites/types of sites?
System manufacturer and model?
You have tried uninstalling, then reinstalling your network drivers?