I created my own batch (cmd) file in Vista to schedule a Boot Time Scan (on next Boot) using the Sched.exe tool Avast has in the application folder. Then I setup a Vista Task Schedule with my desired frequency to have to my batch file run and set a new Boot Time Scan. Remember! it just sets up a scan for the next boot. See the contents of my file below, yes I left in the remarks for my reading pleasure. This batch file has been tested only on my Sony Laptop with Vista Prem. Sp2. You may copy and paste it into your own filename.cmd (or *.bat) file, then add a Task Schedule to run the filename.cmd on your own frequency.
rem usage:
rem sched /A:area [/RA:action [/RS:reply] ] [/archives] - schedules launching
rem area = * or folder1;folder2 …
rem action = delete or move or chest or repair or ignore
rem reply = yes or no
rem sched /D - unschedules boot-time scanner launching
rem sched /F:“file” - schedules delayed file operations (listed in given file)
rem sched /U - unschedules boot-time scanner and any scheduled file operations
“C:\Program Files\Alwil Software\Avast4\sched.exe” /A:c:*.* /RA:delete /RS:yes /archives