stargirl,
+1 for Demache's response.
Even without the driver issues - which are a problem specific your computer BTW - the MTP interface presented by either the 4.1.x or 4.2.x OS revs of the N7 leaves very much to be desired even when it is working. And when it is 100% working, it still does not allow you to operate on it as if it were a hard drive (eg filesystem formatting or file recovery operations) - it "attaches" in windows explorer not like a drive, but more like a camera. (You don't even use the "safely remove hardware" facility any longer.)
I'm guessing that the reason you are getting the unsigned driver issue is because you previously installed an unsigned driver for a different/unrelated USB device that also has a match (in its' installation .inf file) to the PID/VID USB device ID that the MTP facility claims on the USB bus.
The "cure" for this - if you want to avoid unsigned drivers at all costs - is to use your windows device manager to walk through all of your installed USB drivers and try to locate the driver which is claiming the same hardware USB identifiers that the N7 uses. You have to put the device manager into a mode where it displays all the devices AND you also need to set an environment variable to do this hidden/not attached trick. (Google is your friend.) I won't lie - doing this is tedious business.
From there, walk through all candidate devices listed & figure out which one is using unsigned .dlls ... and then uninstall it. Then, replug your Nexus 7 and do a driver install from the internet the usual way.
NOTE: while unsigned drivers are pretty uncommon, you ought to be pretty sure you know what the driver you are going to uninstall actually does. I am NOT recommending here that you go through and uninstall every unsigned driver!
if you want to avoid all this but still want to have a means of doing backups of your internal SD drive, you can use an app which understands network file shares (Windows/Samba) such as ES File Manager & copy things across your home network to backup storage.
As of right now (4.2.1), the only thing the MTP file system interface is good for is read only PARTIAL backups (it won't show you all files and you generally need to reboot the Nexus 7 in order for the MTP facility to show you any newly modified files since the last time you attached the N7 in MTP mode - ugh).
eu1
PS hi demache.
