Since it is a tablet pc, I highly recommend Vista over the previous versions. Having progressed through XP Tablet Edition, Tablet Edition 2005 (XP SP2) and Vista, each upgrade was a giant leap over the previous one in usability.

If your tablet pc came with Vista Business or Ultimate, you have downgrade rights to XP Tablet PC Edition. Otherwise, there is no legal way to obtain XP Tablet PC Edition if your computer didn't come with it. Microsoft released XP Tablet PC Edition only as OEM-installed product. They are not sold through stores.