Please, if you have any data on your SD card - back it up before we start.
You can use something like Win32DiskImager
Using Win32DiskImager, do the following to back up:
1. Run Win32DiskImager
2. Create a file
3. Click the "Read" button
Now just wait for it to finish, then boot your Raspberry Pi.
Note: This process should not affect your data - but it is good form to back up anyway.