Getting an external timer working for cerberus was a lot harder then it should be, especially since the procedures in the install instructions just plain don't work :-)
This is for version 3.2.1 Release (Build 324)
I tried the wget approach, no luck :(
I tried the php approach (btw typo for C:\php\php.exe C:\wwwroot\inetpub\cerberus-gui\cron.php ) the path should be c:\inetpub\wwwroot\) but I would always get, this IP is not authorized to run the schedule, even though I had added 127.0.0.1, tried other masks etc..) :(
So.. the way I finally did get it to work is using the windows port of lynx from here http://www.pervalidus.net/cygwin/lynx/ (at the bottom)
I set up a windows task using "C:\Program Files\lynx\lynx.exe" --dump http://localhost/cerberus-gui/cron.php?verbose=1
with a start in: "c:\program files\lynx\"
You also of course need to set it to external timer in configuration -> scheduled tasks and you also need to add the getmail scheduled task and make sure it is enabled.
Hope this helps someone else out there!