This is an easy one:

1. Connect your Xbox 360 Controller to your PC

2. Wait for Windows to install drivers

3. Locate the "WillowGame.ini" file on your computer
(C:\Users\YOURNAME\Documents\My Games\Borderlands\WillowGame\Config)
NOTE: sometimes the file is called "DefaultGame.ini"

4. Edit the following line:
FakePlatform=-1

You only edit the "-1" to whatever buttons you want to display in-game.

For PC keyboard/mouse:
FakePlatform=-1

For Xbox buttons:
FakePlatform=1

For PS3 buttons:
FakePlatform=2

Save the file and open your game, you will see Xbox 360 button icons on screen.

The point is to emulate the Xbox 360 Controller, it doesn't matter if you use a PS3, Logitech or any other one, the game will "think" it's a 360 controller, for in-game button icons you just edit the FakePlatform=-1 line.

1. Connect your USB controller to your PC

2. Install the drivers

3. Download a tool called TocaEdit Xbox 360 Controller Emulator Application
Download[x360ce.googlecode.com]

4. Extract the zip file into your Binaries folder where your game is (it's the same folder where borderlands.exe is, probably C:\Program Files (x86)\Steam\SteamApps\common\Borderlands\Binaries)

5. Now when you run x360ce.exe it will ask you to match the buttons on your USB controller to the ones on the 360 controller

6. After you are done click on save and close the program.

7. Locate the "WillowGame.ini" file on your computer
(C:\Users\YOURNAME\Documents\My Games\Borderlands\WillowGame\Config)
NOTE: sometimes the file is called "DefaultGame.ini"

8. Edit the following line:
FakePlatform=-1

You only edit the -1 to whatever buttons you want to display in-game.

For PC keyboard/mouse:
FakePlatform=-1

For Xbox buttons:
FakePlatform=1

For PS3 buttons:
FakePlatform=2

Save the file and open your game, you should see the correct button icons in-game.