Switching Hard drives....How to

I have an old hard drive thats only got 5 gb on it that i used to load windows xp on. [b/c of an emergency]I have other drives just needed to use one that i could rely on.

Ill be buying another hard drive soon, or rather my brother will be giving me one of his extras. What do you recommend the best course of action of getting the old hard drive copied to the new one, if its the default harddrive that windows is installed on?