how do you back up contacts for windows 7

You need to sync your contacts to a supported application on the computer. For a Windows 7 computer, that would be Outlook or Windows Contacts.

Read here:

http://support.apple.com/kb/HT1296

Warning: Do not sync to an empty contact application on your computer, doing so will erase all contacts on your phone. Make sure whatever application you use has at least on unique entry before you elect to sync contacts on the Info pane in iTunes.