With all the back and forth of changed drive letters it could have an impact on the avast installation.

Try a repair of avast:
XP - Add Remove programs, select ‘avast! Anti-Virus,’ click the Change/Remove button and scroll down to Repair, click next and follow.

You may need to reboot after the repair.

If that doesn’t resolve it you may need to reinstall avast, a clean reinstall might be best:

  • Download the latest version of avast, 6.0.1125 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).

    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 5.x if previously installed and then for 6.0, once complete reboot into normal mode.- 3. install the latest version, reboot.