Can i use a old HDD with a new motherboard?

I'm not an expert, but you should be ok, as long as you don't reinstall. Windows ties itself to the motherboard, but as long as you have it already installed I would think you are ok.

As for the small HDD, that's actually a recovery partition. If something goes south on the computer, it will allow you to repair the windows install.

Hope it helps!  

i have not got the motherboard/CPU yet. just the case/PSU. just wishing i can just use the HDD the way it is now in my HP, and add it too the new build setup when i get it. or maybe i can do the system recovery before i gut this HP so its fresh.

I would say try it out! I see no reason why it wouldnt, just make sure to uninstall all the old drivers before you swap over.

You always do a format and a fresh Windows install with a new motherboard, ALWAYS, or your bound to have troubles. The drivers on your hdd are for the old hardware and will not work with your new hardware.

Even if you have a problem installing Windows because you installed a new motherboard, it only takes a 5 minute phone call to Microsoft's automated line (the number will come up on the screen during install) and get a code. You don't even have to talk to anybody.

You do not have to buy a new OS. I use Vista64 myself and love it, just install Windows, not any of the HP junk utilities or drivers, they will not be the same drivers that you need for your NEW hardware.

Thanks for the correction Geek. as I said, I'm no expert in the area and have never been in the situation where I had to swap out the motherboard. I'm still somewhat of a noob  

Im lost... im not even close too being a noob, im a supper noob times 1000.

I don't have a vista CD, just the lil mini HDD with the HP recovery tool/factory vista install. But that adds all the HP software as well i think, or will it give me a option to let me install vista without the HP software?

I've built every computer I've ever owned so I have no idea. Only you can answer that question. Find a friend or relative that can help you out.

You might want to be prepared for it to tell you that you are trying to install an OEM version of Windows onto non-OEM equipment and fail.

Hmmm...but don't you own the right to the Windows copy if you have a cd key?

I would think you could call them and tell them you are replacing the motherboard and hard drive.