My friend showed me on her laptop how she can tell when someone turns on her laptop & even when someone attached a little thumb drive & copied files to the drive. I have tried to find this on my computer (a Dell PC with Microsoft XP Professional) but I can't find this information. I think someone is logging on to my computer & copying files. I can't password protect it because I share it. I believe this happens after we have gone. I'd appreciate if anyone can tell me how to solve this mystery.

How do you mean? Do you want to know if someone hacked your computer?

Put a logon password that only the people who use it know what it is

My friend showed me on her laptop...

She may have specialized software. In any case, you can view the Event Viewer via Start > Control Panel > Administrative Tools. Select the System group and there will be a list of information and probably some error items. Double-click on an item to view the entry. From these you can at least see a pattern of dates and time of when the machine was started and stopped.

Look at the System Event viewer in Admin tools. This will definitely show you when the computer was used last.

I just password my files with 7zip or winrar

First you should put a password on your logon then you can verify with the event viewer,to password all the files are OK but when you share a PC is not so practical operation,because you need the write the password every time you want to do a operation copy/move so on.

Win logo+R
>type
Eventvwr.msc
>enter
>view PC usage info.

My place was broken into. I keep my email and face book logged in... I went to the browser history and it shows activity for the time my room was broken into... does this mean that they turned my computer on and viewed the contents that are showing the activity on this date as well?

So sorry to hear this terrible news. My home was broken into in the past. If search history was recorded during the time you suspect this break-in occurred, the person(s) who broke in used your computer and navigated the web.