First thing - locate that "Dr. Who" file and move it to your desktop for easy access.
Next: Plug the USB end of your charger chord into your computer, and the other end into your phone.
On my Windows 7 machine, it takes about 20 seconds for the system to recognize and shake hands with the "new" device.
Close all pop-up windows and open My Computer - where you should now see your phone listed as a newly-available drive.
Double click on that icon and it will open up the inside file tree of your phone, where you will see a folder named "Ringtones".
Find that "Dr. Who" file on your desktop and simply drag it into the Ringtones folder. In some cases, you may receive a message that the Android system on your phone needs to convert the file into a different format and, in that case, click "YES" and proceed.
If you double-click on the Ringtones folder, you will see the file in the folder. You will not see the pre-installed ringtones - however, if you open the ringtones list on your phone, you will see the new ringtone listed in its alphabetical place - and you can test it from there. You can do that while the phone is still connected to your computer.
If you look on your desktop, you will see the original "Dr. Who" file is still there, which you can save if you like.
To remove the ringtone, re-connect to the computer, open the ringtone folder, locate the ringtone you wish to remove, and simply delete it as you would any other file - or you can drag it out of the folder and back to your desktop.
I've done this many times and have had great fun installing ringtones - both custom and pre-made...all for free.