Please explain more about your problem and any errors you are getting - if you are using Windows Live OneCare. If you are not using OneCare, you're off topic for this forum.
What is the operating system on the PC?
What is the specific error you are getting about "won't let me backup?"
Where are your trying to backup to? CD/DVD, External drive, network location?
Assuming that you are using OneCare, is it your subscription or trial that you installed or was it on the PC when you got it?
-steve