It could be as harmless as having another local user logged in and forgetting about it, all the way to your neighbor being way too lax about security and someone having compromised the system as a result.
If they can't determin that it was indeed just another local user account being logged in that was forgotten about, they should backup everything they want to keep from the computer and reformat. This time making sure to turn off Windows file sharing, making sure to install ALL of the Windows Updates and to keep up to date with them, keep your AntiVirus software up to date, spyare scanners, and maybe even install a firewall. Make no mistake, the firewall WILL NOT stop someone from getting into the system, at best it will slow them down and give you some sort of notice of the attempt. Maybe even a log of the IP address which can be forwarded to the proper authorities to be officially ignored.
But if someone has been wandering around in their system without their knowledge, there's no telling what has been done to the system at this point. The easiest way to be sure there's no little nasty surprizes left is to reformat and wipe them all out. Tell your neighbor to remember how annoying this task is, and to use it as motivation to be a little more vigilant.
Received this message from neighbor:
I was using my computer before supper and let it on while I ate. I came back at 8:00 PM and decided to shut it down. Here is what happened:
When I went to shut down, I got the following message:
Other people are logged on to this computer. Shutting down Windows might cause them to lose data.
Do you want to continue shutting down? Yes / No
I hit Yes. Computer goes to Windows XP, but will not shut down. Had to push button to finally get it to shut down.
Got any ideas why this would happen?