Go to install programs (or however that is called in VISTA, formerly add/delete programs in XP Control Panel), select avast, select "add" and then add the Mailshield.
If it is already there, do a "repair" installation.
If that won't do: uninstall and boot to safe mode, run aswclear5.exe (downlodlink in my sig), search for all "alwil" and "Avast" folders on C: and delete them.
Reboot to normal mode and install from scratch. Turn off all Email clients before installation.
REBOOT.