it's very easy to do. but like Shannon said.. how many files do you have? you might even be able to make a CD/DVD or use a USB stick and Xfer the files if you don't feel comfortable taking apart the system.
Do a google search for replacing a hard drive and research. if you can find one with pics, it'll make it even easier. Windows should see your hard drive and install it for you without any problems, then pull from one drive to another.