Restore Vista on HP desktop
I have an HP Pavillon a 6220n desktop. What I did to go back to Vista was to press the F11 key when you reboot and restart your computer. Your OS boot menu should have it there; like ESC to access the menu, etc., and F11 will say "Recover my computer". Mine did not have an OS disk or CD either. It is installed in a separate partition of your hard drive and the F11 key will invoke this recovery. You can also go to HP.com, customer support, type in the model of your computer and you will have an option to order the recovery cd if, in case, the F11 key does not bring this up.
Hope this works for you.