As you know,windows 7/vista put all system data and user data on the same partition,normally it’s drive C.Under this condition we must backup our user data every time we re-install our windows 7/vista.From windows vista ,Microsoft supply us a tool mklink which can create a symbolic link under windows 7 or vista just as the command ln under unix/linux.Below is an example about how to put user data on non-system using mklink in windows 7.
1.Enable administrator account and set a password for it.You can refer How to enable or disable administrator account in windows 7

2.Logoff and login with administrator account

3.Relocate your user data to other non-system partion,for example from C:\users\J  to D:\users\J

4.Use mklink to create a symbolic link from C:\users\J to D:\users\J

5.Re-login use your own account,you will find nothing changed,but in fact your physical user data is on drive D.

6.Done.The next time you re-install your OS ,just use the same user name, remove  C:\users\J then run the command of step 3 again then your user data is back.