The file that you are referring to is the OneCare Event Log, the service that is locking it down is the "Event Log Service".
I don't think that you can stop the Event Log Service, so I think the only way to remove it would be to use the cleanup utility like Steve suggests.
Worst comes to worse you could try booting into Safe Mode or even into Recovery Console and try to delete it from there, both of which you may want to get help from support as they are pretty advanced steps.
Hope this helps.