Are you trying to download and run in one action and not save the download to your hard disk and then install it later ?
You also appear to have had this problem before,
http://forum.avast.com/index.php?topic=73110.msg610376#msg610376, can you recall how it was resolved ?
If not try a quick browser through that topic again.
EDIT:
Though it seems that topic was only loosely related (as in you had the same version), so it may be best to do a clean install:
- Download the latest version of avast, 7.0.1426
http://files.avast.com/iavs5x/avast_free_antivirus_setup.exe and save it to your HDD, somewhere you can find it again (if you didn't save your last download). Use that when you reinstall.
- Download the avast! Uninstall Utility, aswClear.exe
find it here and save it to your HDD (it has uninstall tools for 5.x, 6.0.x and 7.0.x).
- 1. Now uninstall avast! (using add remove programs, if you can't do that start from the next step), reboot.
- 2. run the avast! Uninstall Utility from safe mode, first for 6.x if previously installed and then for 7.0, once complete reboot into normal mode.
- 3. install the latest version, reboot.
Programs & Features, Uninstall/Change
