If you're having trouble with your Windows Phone 7, here's how to reset it to the pristine factory settings you had when you first got it.
Please note that this will clear all data and will reinitialize all settings.
1. Go to your Home screen and tap the Application Menu Key.
2. Go to Settings then System then About.
3. Tap on the "Reset Your Phone" button.
Now your Windows Phone 7 will be back to its original factory settings.
[Windows Phone 7] Reset your phone to factory settings | Kioskea.net