The history is stored only under the user profile, so theoretically, once they are cleaned they are gone.
You can do logging at the router/gateway level - most newer products have the feature, but you have to have a machine ready to receive the logs.
I can do this on my Linksys wrt54 at home and I know all the other products on this line have this feature.
Ricardo A. Parodi
eSolar, Inc.