Deano - you certainly can! Give it an annual dressing of fresh compost and repot whenever the roots start to grow out of the bottom of the pot. Depending on the species you have (might be some sort of dwarf spruce), it could grow to around chest height.
Teresa - in theory, yes, you can, but the results may vary. It will have a better chance of survival if it was pot grown, rather than having been grown in a field and then moved to a pot for sale. You'll need to prune the roots as well as the branches, and, eventually, it may lose its traditional Christmas tree shape. No harm in trying though!
Kate