did u have any older versions of avast prior to v7? eg v6, v5?
if u have any of the previous versions, u will also need to run awsclear on them for each version, with a reboot into safe mode for each time.
u should also try to re-download the installer to prevent possibly a corrupt downloaded installer.
if the above does not work, please provide a screenshot of the error, system specifications (eg. windows 7 32bit, sp1)
please wait for avast developers to help u diagnose the problem.