edit: while we're at it, I didn't think you could even run IE6 on vista...
You haven't installed XP to the Virtual PC yet.
1) Make sure you've got VPC 2007 and install it.
2) Make a new Virtual Machine (click New on the miniwindow).
3) Click Next until the end, only stopping to set the VM os type to "Windows XP"
4) Double click the new VM.
5) It should run a couple of Bios-y things and then stop with "Please insert Boot device".
6A) Put an XP disk in the drive and select CD from the menu followed by the drive letter.
6B) Select CD from the menu and select the XP ISO file
7) Select Action -> Reset
8) Follow the instructions to install XP - you'll need an XP licence key to do this.
9) Once finished, and you boot for the first time, make sure you go to Actions -> Install Or Update Virtual Machine Additions - it speeds everything up by about a factor of 2.
10) To exit, ask the XP virtual machine to shutdown, rather than closing the VM.