well if its one or two big problem-crumbs i open up Microsoft Word (so it doesnt matter what keys you press) and hold down the keys where the crumb is to see if it will push out. This isnt expert advice and im not sure if this is very handy or good for the computer but it works for me!!! If i get a row from a computer expert on here i will learn my lesson!
Use a hoover, and simply suck them out. I wouldn't recommend trying to prise the keys off a laptop, with a desktop keyboard they tend to pop on and off but laptops can be different.
First of all, turn it upside down and give it a good shake to get the worst out.
Then nip down to somewhere like PC World (or even Tesco) and get one of those aerosol cans of compressed air with the little plastic tube to stick in the top. Then just poke it between the keys and squirt!