As Shilpa says there is no way to do it, however you might want to keep an eye on this thread
http://forums.symantec.com/discussions/thread.jspa?threadID=65245&tstart=0
which is discussing the exact same thing, in case anyone manages to think of a successful work around.
Alternatively, since the logs are just xml files, why don't you periodically archive the old ones to another location to clear space from your c:\. Not as tidy admittedly, but at least it'll allow you to keep the logs without running out of space.
Keith