You can't use a Playstation 2 Controller as an xbox 360 controller on pc version of left 4 dead, can you?

If you have a PS2 to USB adapter box (like the Joybox Adaptors), then yes.

I have a Joybox adapter, and played almost all games with a PS2 controller, until I got a Xbox 360. Now I use the Wireless gaming reciever to play games on the Xbox controller, since this controller offers the analogue R/L triggers.

First, no respectable FPS gamer uses gamepads. At all. Using a gamepad for FPS games puts you distinct disadvantage against players using mouse+keys. You will not be able to change direction as quickly or as accurately compared to using a mouse to do the same tasks. Try playing some single-player mode games to get used to a keyboard if you aren't already.

If you want to hook up a PS2 controller to a PC, you will need a USB adapter - they can be had for pretty cheap on eBay and other online shops (they aren't usually found in real-world shops). If you want a gamepad to work with any game regardless of that game's level of joystick/gamepad support, use an input emulator such as Joy2Key or Xpadder.

No. Get a PC controller

nope. a controller with a usb plug would work though.