1. Contact HP and buy a (replacement) Recovery DVD. Most HP's let you burn your own DVD the first time you use it, just in case your hard disk dies, but not everybody does it.

2. More expensive: buy a retail or OEM Vista and install that on your new hard drive. Download the drivers from the HP site; you might need to contact Customer Service about that.

3. About as expensive, but takes 2 months. Buy a retail or OEM Windows 7. For the drivers, see #2.