I am not sure why felix said that you could not restore from the image, but he may be right. Maybe you have to retore back to the system/drive you made the image from--even when using True Image. My impression is that keys and disks, whether OEM or not, are not so tightly linked that one cannot use the Vista DVD of another along with his or her key. Also, just in case, are you aware that you can get student upgrade versions of Home Premium for about $70--for example, here:
http://www.academicsuperstore.com/. So, whatever happens, you are out no more than $70. Be aware that if you upgrade XP, then its license is no longer valid.