Avastclear tool http://files.avast.com/iavs9x/avastclear.exe, run the tool and it will ask to reboot to safemode which you need to accept and when finished reboot back to normal mode.
Also run CCleaner disk and registry clean/reboot and then re-try your install.
You say the download of avg is failing, but in your quote you say the installation is failing.
Which one is it?
Got any problems if you try to install another application that uses the msi?
What OS/SP?
Any anti-malware software installed prior to avast, and/or now?
Downloading is not the same as “loading”.
Loading is when you (try) to start a already installed application.
So, at what exact point is it going wrong/are you getting the error message?
Please mention all steps you are doing like:
going to the avg website (exact website link)
clicking on the download link (and what exact link)
etc. etc.