That's kind of correct... modifying inputs.conf is the way to go (make sure you restart Splunk to apply the changes), however your config is slightly wrong, you should try:
[WMI:WinEventLog:Security]
disabled = 0
start_from = oldest
current_only = 1
This will just read from the oldest to newest (i.e. normal cronological flow), but only starting from the first event, from when the input is enabled (i.e. restart Splunk to apply config). At the moment you have it backwards, as it will read in reverse chronological order and then go back to the latest, which doesn't make sense.
HOWEVER... although you may exceed your daily limit, Splunk will not punish you for you first offence, as in a real world there are times when we find the unexpected, and perhaps your logging source goes rogue and pumps out gigs of data. You should read the following doc, as indexing some of you older/historic events may be useful in showing Splunk's potential...
http://docs.splunk.com/Documentation/Splunk/5.0/Admin/Aboutlicenseviolations
Hope this helps
... View more