Dual Boot XP after Vista Ultimate

I have read many suggestions on this topic and apparently there is not a simple one size fits all situations. Originally I had Vista Home Premium and Microsoft talked me into upgrading to Ultimate in order to dual boot (so I purchased the Ultimate Upgrade Disc) - then they told me that XP will not install into a virtual drive (partition?). Regardless, I created a partition on my hard drive and tried to install XP but the install did not recognize the partition and only wanted to install on "C", needless to say I canceled the install.
My computer is a new HP Pavilion M8100N and has a SATA drive and I have read that I need an XP SATA Controller driver but I don't know what this is or where to find one. Any help with this?
It is really important that I install XP since my Autocad version will not run on Vista, nor will my Quickbooks or my HP Plotter, all of which I now have to run on my notebook XP. Upgrading this software and hardware will cost over $2000.
I have read about VistaBootPro which would seem to be helpful to control the Vista boot after XP is installed.
Can anyone help with step by step procedure? Will I ever be able to get the dual boot running? Will the computer be stable when I am finished?