The Windows edition listed in BIOS (Vista) is referring to the operating system that came with the computer. BIOS's System Diagnostics will always say the OS is Vista, because that is the original OS that HP put on the system. BIOS itself, however, cannot detect an operating system on the new Hard Drive because there is
no operating system. In order to get an operating system, you have a couple options:
Depending on the terms and length of your warranty, you may be able to get HP to reinstall the operating system back onto the new Hard Drive. The amount you would pay depends on the warranty. Many warranties, however, become void when you change physical components in the computer (especially with laptops). If this is the case, you will have to try the next option.
You can buy a retail copy of Windows either online or from an electronics store. The price will depend on what edition you purchase, but it will probably cost you at least $100 US.
You can install a Linux operating system on your Hard Drive. If you've never heard of Linux, it is an operating system based on much of the same code as Mac operating systems, but has a more "Windows-like" feel to it. Linux operating systems are NOT the same as windows, and some programs that work in Windows will not work in Linux; you would need to find the Linux counterpart for these programs. One of the best things about Linux operating systems are that they are absolutely free to download, install and use. A very popular and stable Linux operating system is Ubuntu, which can be found at: http://www.ubuntu.com/