If you use Instant Messaging, there can be logs, or you can disable them. You can password protect your user account, and you can encrypt files of your choice. The event logs will tell you when someone boots the computer, but not much about their activity. You could look in temp files and MRUs (most recently used) to see where they have been. Tell me what you specifically want to protect, and I'll tell you how to keep someone out. That is easier than trying to figure out where they have been.