Yes the hard disk can be upgraded. Check the computer specifications in the User's Manual and see what the largest hard disk size that the motherboard will support. Then buy this sized hard disk from the computer store of your choice.
Next, the existing hard disk must be 'cloned' to the new larger hard disk. Get this product for cloning: http://www.apricorn.com/product_detail.php?type=family&id=39
Follow instructions. I have done this with my laptop. Works very well.