I was lucky as my machine came with an oem version of vista.
I had to delete the massive "C" drive and run a third party software ( killdisk )to totally wipe the "C" drive.
I then ran the XP software to create the first operating system. I partitioned the "C" drive as it installed XP. I then loaded the drivers for XP which I had previously download from the HP/Compaq site.
I then booted from cdrom and then loaded Vista on the other partition. Then loaded the drivers.
It now gives me the option to boot into an older version of windows (XP) or to boot into Vista.
You can pick which one you want as your default O\S.