How can I tell if someone turns my computer on & uses it after I've shut it down? Is there a history of when it is turned off & on & what files, folders, or programs they access? Can I tell if they attach a thumb drive & download or copy files to this external drive?

Yes, you can find out if your computer has been used. What kind of computer and operating system do you have? It would be helpful to know that in order to give you the best answers here. Is there a particular reason that you are having concerns about the usage of your computer? I'm not sure about the inquiries concerning a thumb drive.

I have a Dell PC with MS Office XP Professional.
My friend showed me on her laptop where someone had logged on then she inserted her thumb drive & copied files to it. She showed me how her laptop showed an external drive in "J" drive & which files were copied to the drive. I have looked for this on my computer but can't find it.

Carol,install a local keylogger and set it to work in hidden mode. The keylogger will save everything typed/websites visited while you were away.

Keyloggers and something called CyberPatrol can go and basically video tape your monitor and you can play it back like a VCR. Handy for parents like mine. Too bad I can outsmart it.

Create a User Account for yourself with your own Username & Password so no one else can log in.

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Control Panel
User Accounts