How can it be the problem when you state, when you disable Web Shield your internet doesn't work, surely it would be the reverse, when it is on it doesn't work?
When you say you have disabled the Avast WEB Shield Service, are you talking 'Windows Service?' see image. If so that could be the problem as the web shield 'provider' if running your browser would need the windows service otherwise it can't work.
If you can restore what settings/services you have disabled then try a repair of avast. Add Remove programs, avast! Anti-Virus, Change/Remove button and scroll down to Repair, click next and follow. You need to be on-line to do this.
If that doesn't work try, uninstall, reboot, install, reboot.