Windows 7 - Guest account validity...

My Windows 7 pc has never been shared or set up to share with anyone. Occasionally, when my pc is chugging away doing something when I'm not using it (which it always is), I right-click on a program icon and under the 'security' tab, there is an S-1-25, etc. group name or user account. Before I can right-click on the account to try to find out who or what it is, it disappears!

I've been trying to look into this lately to see if someone's hacked into my computer to explain why it's so busy all the time.
Now I've gone into my User Accounts-Manage Accounts area, and I see a Guest account that I never created, that is apparently turned off.

1) Should it be there?
2) If yes, should it be renamed or have a password attached to it?
3) If no, should I disable it for security reasons?
4) Does it have anything to do with the disappearing user account I see all the time?

For my peace of mind, I'd like to know more about this this. Thanks!

1- Yes
2-No it should be disabled unless using it.
3-Yes
4-No.

The mysterious GUID account probably is just system, or it may be your GUID (globally unique ID)

Thanks very much for the quick reply!!

My pleasure. It is good that you are keeping an eye out. Good luck