I did this last night and found I preferred using it to the keyboard as I found it more fun to lean back in my computer chair while I played, also liked the way swapping spells worked better and it seemed more intuitive to me. I also don't own the software used (Motionjoy and DS3 Tool) and can't claim it's 100% legit (seems like it's from China) but it works just fine.
Originally found from here, but the instructions are outdated and didn't work right away for Skyrim.
This worked on my Win 7 x64 bit install.
1: Plug in your PS3 controller via USB, your computer will automatically install some basic drives.
2: Download full drivers (digitally signed, so no worries there) x64 x86
3: Run the .exe in the .zip. When it's done, open the DS3 Tool as prompted.
4: Tool looks like a clown college, but is actually pretty simple
once you get it down. It should automatically prompt to update, if it
doesn't, there's an option to update under "Options" I believe, after
it's done updating it wants you to reboot, go ahead and do so. Leave
your PS3 controller plugged in.
5: Computer's back up, open the DS3 Tool, click on the "Driver
Manager" section, checkmark the box there and click "Load Driver".
6: Click on the "Profiles" section. Under the "Connected game
controller(s)" it should now say "1. Dualshock 3/sixaxis (USB)" (or
whatever controller you have). Under the "Select one mode:" section
check the "Xbox 360 Controller Emulator" - need to do it this way as
Skyrim only recognizes 360 controllers.
7: Click the pink "Enable" button below. More drivers will install,
wait for confirmation notice from your notification area and then you
should be good to go. Click on "Game Controller Panel" and confirm
"Controller (Xbox 360 Wireless...)" is listed, highlight it and click on
properties; this will bring you to a testing area. Click around with
your controller to confirm it works properly. Press "OK", "OK", and then
click the "Vibration Testing" button, vibration should work (if you
have a dualshock).
8: Open Skyrim and it will detect the controller right away, continue
your game, hit up the settings and remap to your liking (I don't like
the 360 layout, I prefer my right and left 'attacks' to be R1/L1, not