I'm not sure how to explain this properly, but i have a freeagent driver and i want to make it my main drive?

You know how you have your local disk C: normally, where all your stuff is saved. well mine on my desktop has 40 GB (my PC is 5 years old, and still has Windows XP so it runs kinda slow) and this freeagent driver has 500 GB, and I would really like to switch to that. Is there any way I could do this?