Microsoft Windows Vista Home Premium Upgrade (Educational) -
You may need to prove your academic status.
"Proving" your academic status in the past has meant you got sent a voucher and got an academic institution to validate that voucher. You were able to purchase academic versions if you worked in a school or university or even just had children attending school. However some interesting comments in the reviews of this product - "Even though I am a student, there is no check for academic status and this is most certainly the cheapest way to get Vista!" however it is definately worth checking this if you don't qualify.
Also there is a workaround for the upgrade edition to do a clean install without having to install your qualifying OS first: http://www.dailytech.com/article.aspx?newsid=5932