The Drivers CD is just that. That is for use when you use your own OS CD/DVD.
Here's a link about what you were supposed to do when the machine was new -> http://h30499.www3.hp.com/t5/Notebook-HP-ProBook-Compaq-Slate/HP-6735S-Vista-how-to-create-a-recovery-CD/td-p/1088021#.UEJqY5tI7Dc
If you opted not to make said media, head to HP.COM and order it now.
The hard disk on my son's computer died. We bought a new one and fitted it. When we switch on it says "non-system disk or disk error. Replace and strike any key when ready". It does this whether the CD drive is empty or contains the original HP "Application and Driver Recovery DVD". What should we be doing?