Sometimes program installations can be corrupted through no fault of the user. Go to Control Panel > Programs and Features and see if Avast is listed. If it is, uninstall it. Then use the official Avast uninstall utility to make sure all traces of Avast are removed. I'd also run a Disk Cleanup and delete any unnecessary non-system files (Step 4 in tutorial) just in case some Avast temp files may remain. Then use the official Avast download site (or your install disk if you're using the paid version) to reinstall. Hopefully this will solve the installation problem.
NOTE: If you now have, or ever had, any other antivirus programs on your machine you should consider running that product's uninstall utility. Previous remnants could interfere with this installation.
