Yes — what you want is called tethered shooting. With a Canon 450D, you can connect the camera to your laptop via USB and use Canon’s EOS Utility, which typically came with the camera/software bundle. It can detect the camera, display Live View, let you trigger the shutter from the computer, and often control settings such as exposure. It may also let you save images to the camera, the computer, or both.

Another option mentioned is Adobe Lightroom, which supports tethering for some cameras, but compatibility depends on the camera model and Lightroom version. For a Canon body, EOS Utility is usually the simplest first choice.

So in practice: connect the 450D by USB, turn the camera on, open EOS Utility, and use its remote shooting/Live View features to compose and capture from the laptop.