Unusual Vista Question

For Christmas I bought my mom a new computer that came with Vista Home Premium.  However, through my school's website students are allowed to download various version of Windows including Vista Ultimate and so I thought it would be nice to put that on her computer.  Really I just wanted to do a clean install and get rid of all the junk software that came with the computer.  What I didn't realize at the time was that I had already installed Vista Ultimate 64-bit on my desktop and they apparently had the same serial number.  Now when her computer loads up I get a menu that says the 30 day trial is up and I need to enter an activation code.  The strange thing is I've tried entering a different activation code for Vista Ultimate from my roommates school account and it won't accept it.  I'm assuming the activation code has to match up with the installation CD that I initially used.  My mom doesn't "need" Vista Ultimate so I decided to download Vista Enterprise and boot her computer by disk but this isn't working either.  I get an error when I plug in the activation code.  I've come to the conclusion that I need to format her hard drive to get rid of the original Vista Ultimate installation but I'm not sure how to do that without being able to get past the initial menu saying her trial has expired.  All I can do is access the internet from her computer.  Is there a way to format her hard drive from the BIOS or Boot menu during start up?  The only other thing I can think of is to take her hard drive out and put it in an empty drive bay in my computer to format it but that would be a pain in the rear.  Any suggestions would be greatly appreciated.