Do you even have a valid connection before installing Avast?

Can you reset the firewall?

If you happen to clean again Avast (with its removal utility from http://www.avast.com/uninstall-utility), then please do it using Windows Safe Mode. If you used to have previous versions (like Avast 5.x before Avast 6.x), then please run the removal utility twice, once for 6.x and one for 5.x (all under Windows Safe Mode).

I would suggest skipping the “Add/Remove Programs” altogether, and running the removal utility directly, and from Windows Safe Mode directly (NOT trying first normal mode).

Then reset the Windows Firewall (there is an option to do it with a simple click).

When downloading Avast setup, please re-download anew and from the official site only. Use the latest stable version.

Before installing Avast, test you connection, so we can narrow down the problem.

Please double check you have all Windows Updates installed. Then reboot and check again for new Windows updates, and repeat until no new Windows updates are offered.

Some Windows updates will not be offered until Windows is validated. Double check it is (since in some situation a re-validation is needed and the user haven’t noticed it), reboot and test Windows updates again.

In any case, please report back.