Just had my mobo and psu die on me so I bought new ones. I'm assuming that my old processor still works (fingers crossed). Now, after I install them what is the next step? In other words, do I still have to install the operating system even though my old hard drive has it already? Are all of the files from my hard drive still going to exist after I install the OS (if I have to) on the mobo? I'm thinking there's an option if I have to install the OS where I can keep my files on my hard drive correct? The only things that are new in my system will be the mobo and the psu...
You should be able to get it all working without a re-install. But it's unlikely to just work when you hook it up.
First problem likely will be that the Windows (I'm assuming!) already installed on the hard drive does not have the right device drivers for some of the things on the new mobo. That can be fixed, I believe, with a Repair Install of Windows from the original install disk. NOTE this is NOT a re-install! A Re-Install might wipe out existing stuff you need! I have not had to do this, so I recommend you search around for how to do it. It MAY require booting in Safe Mode and deleting all drivers, then rebooting to force Windows to search the original install disk and find drivers for everything. Sometimes the mobo comes with its own driver CD.
After that is working you certainly should run a Windows Update to ensure all the items newly installed are the latest versions. (You might even need updates to drivers for other devices like DVD drives, in case the ones on the original install disks are outdated.) Right here (maybe before) is where you could run into Microsoft's software piracy preventers. Your Windows may decide that there is enough stuff changed from before that it no longer believes it is installed in the original machine for which it is licensed. If it refuses to run, or hassles you during updating, you may have to contact Microsoft support and explain what you changed and why. Once they are convinced that you just repaired your machine, and did not try to install your Windows on an unauthorized second machine, they can help you re-authenticate you Windows if needed.
Insert your motherboard cd with new drivers when the windows screen appears. Windows will start rearanging devices as soon as it loads; if it doesn't load, you'll get a blue screen, which would indicate a fresh install is in order. I did a repair option with vista, but it didn't work until I rebooted and loaded the motherboard bios defaults. I had to boot up with the vista cd to use the repair feature.