As others have said it depends how far you are in the background, pixellation will happen on every image if you go in far enough and there's not a great deal you can do to improve it. If it's not your photo then you might contact the person that gave it to you and see if they have a higher quality version?
Mildperil's linked video sums up the misconceptions about image enhancement. I'm not a guru with Photoshop, but I am competent, and the bottom line is that if I want to zoom an image to the extent that it becomes pixellated then I need to use artistic licence in how I manipulate the image. The bottom line is that much of the nonsense you see on TV progs when they zoom in to a ludicrous degree on digital images is utter nonsense.
There's a slack handful of variables which will dictate your answer; What is the photographic quality (ie is it sharp and correctly exposed?) of the original image you are working with? What is the file type and size? How much do you need to crop? What size do you want the final image to be?
Whatever, cropping an image will reduce the quality, you can just get away with more of you've got a decent pic to start with. This can be mitigated to some extent by using software like Photoshop to resize the cropped image, it does this by interpolation- creating new pixels using the information from existing ones- but you can only get away with a certain amount before the image goes 'smeary'.