If HP deliver their computers without an OEM OS install disk, then that's a real shame indeed.
Since you already have a legal copy of the OS installed, and you only need the disk for the purpose of repairing what you already paid for, I guess it will be legal to borrow one such disk from a friend, colleague or neighbour.
Once you got hold of one, here's a visual guide on how to repair the Vista bootloader by means of that disk:
http://neosmart.net/wiki/display/EBCD/Recovering+the+Vista+Bootloader+from+the+DVD