If you want to assign dedicated ringtone for a contact on your Windows Phone 7 mobile phone then follow the process written below.
Assign / Change Ringtone for a Contact
On “Start” screen, tap “People” and flick to “All”
Tap on the required contact to which you want to assign ringtone and then tap “Edit”
Tap “Ringtone” and select the ringtone which you want to assign to contact then tap “Save”
Done! Your selected ringtone will be assigned to that specific contact.
Note: To assign separate ringtone for each of your contact on Windows Phone 7 device, you have to repeat the process for each one individually.
See more Windows Phone 7 tips