Do you mean you are purchasing a PC with a 40 gig drive or just a 40 gig drive as you wrote? If so, are you wanting to just transfer a larger drive with XP and all software and data intact? This will affect the answer as this part gets complicated. Much depends on the software disks you already have and those that will come with a new PC. Yes, you can change the drive with one from another PC. There isn't a physical size difference unless your small footprint unit uses a notebook sized 2?" drive. Both also need to be the same interface type and I will guess IDE. A little more about your exact plans would help as such a transfer with already installed software is not plug and play.