Okay so, i had the same problem too. i reinstalled my windows and i was unable to get my itunes to backup my iphone. Luckily, before reinstalling the windows, i had saved the backup files of my iphone.
1.You can do this by going to: C drive/users/user/appdata/roaming/applecomputer/mobilesync/backup.
2.Note: windows computer do not show hidden files by default and appdata files are hidden.
to see the hidden files, visit http://windows.microsoft.com/en-US/windows7/Show-hidden-files
3. so next time you format and reinstall the windows, make sure you have backed up the iphone backup files.
4.so now after installing windows again and after reinstalling all the essential softwares, reinstall itunes and connect your iphone to itunes and authorise the computer.
5. go back to the iphone backup file storage location which is:
C drive/users/user/appdata/roaming/applecomputer/mobilesync/backup.
6. once you are there you will see that there are no back ups in that location which is quite obvious. now paste the backup files of your iphone ( The backup which you made before installing the windows). as soon as you do this, you will see that the tab whi read: Restore form a backup glows up and is Clickable.
7.so now, you have the backup and restore fully functional. for a safer side, make a new backup and transfer all the purchases to your new itunes.