you need to provide more and detailed information about your problem or what you want to get done.
We don't have a clue about what you are "rubbing out" and nothing "fills up" when a computer is switched off.
Sounds like you have backed your data up on a pen drive and that is good as long as you know that the pen drive can fail too - it is recommended to have at least two and preferably three copies of current backups with one kept off site so it is still around in event of computer or drive theft or fire or flood etc.
To back up your emails using Windows Mail here are some pretty clear instructions:
http://www.vistax64.com/tutorials/90298-windows-mail-messages.html
Now, just give us detailed info about your actual PROBLEM (such as make and model of computer, Operating System and version (basic, home, pro, etc.) and highest Service Pack applied, and why you think you need to re-install the OS.
As I rub things out it just fills up--- sometimes just does it while the computer is swithched off. I have put all my valuable stuff on a 32 GB usb stick. I would really like also to keep my emails that are all in folders on windows mail but don't know how to do this or how to find out how much space they take up. Then I suppose I must reformat or clear everything out and reinstall my system. I don't know how to do this yet.
Thankyou for the help these forums have already sent me.
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