I don’t know if your failed attempts to update haven’t had any impact on your installation of 5.1.889, so it would probably be best to do a clean install of avast 6.0.1125:
Download the latest version of avast free 6.0.1125; you didn’t say what version you are using so I will assume that you mean the free version http://files.avast.com/iavs5x/setup_av_free.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 both 5.x and 6.0.x).
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 5.x if previously installed and then for 6.0, once complete reboot into normal mode.- 3. install the latest version, reboot.