I was presenting at a Partner event on Friday, and in the Q&A session at the end, someone stumped me with a question about the difference between the Prefetch in XP, and SuperFetch in Vista...
Well, there's a good answer here: http://news.softpedia.com/news/SuperFetch-Enhanced-Prefetch-64346.shtml
In essence, it's around intelligence. On the surface, they are both ensuring that applications load more quickly, but SuperFetch ensures that in the long term, it ensures the performance of your PC will stay tip-top thanks to it's intelligent management of your use of the PC, files, folders and applications.
"To start with, SuperFetch overcomes one of the big drawbacks in Windows XP's Prefetch technology. Prefetch improves efficiency by loading the majority of the files and data needed by an application or process into memory so that they can be accessed very quickly when needed. However, because these files and data exist in memory, they are subject to the laws governing virtual memory. In other words, when other applications need access to memory, any prefetched data is moved out to the page file on the hard disk. When it is needed again, it then must be moved back from the page file to memory, which of course offsets the performance enhancement" (http://members.rushmore.com/~jsky/id37.html)