Bassfishers advice is wise, if you are not familiar with the method, but I have done it several times with Eusus. I believe there are other third party programs around which will also complete the operation.
For normal desktop operations, 75gbs is rather lean by todays standards, but that depends on your usage. If you intend to play modern games, you will quickly be out of space.
But, I assume, if you are going to buy another HD, you have the Windows installation Media? Perhaps you could back up your private data to a DVD, and, during the beginning of the install, accept the options to delete the smaller partition(s)