Windows Carry-Over?

I am about to build my first pc, but since money is a serious limiting factor I have a question about my operating system.

To save money I am going to use the HDD from the pc I have now. Will windows transfer over to my new pc in my HDD?

I have the product key for it but I don't have the windows recovery/reinstall disk (not really sure what it is called) from when I originally got this pc. Just trying to save a few extra bucks in my new pc. Any advice is very much appreciated.

if you have the windows keycode just go Here: http://www.w7forums.com/official-windows-7-sp1-iso-image-downloads-t12325.html download the one that matches your key and burn a image of it and reinstall it on the new machine.

Don't try to migrate your os to new hardware, as the old drivers will make your life a nightmare. Do as Nitrodemon says and download it, however if you have an OEM copy, you will either need to buy a new copy of windows, or download linux.

The above statement is not entirely true. You can use the same copy, just have to call them maybe and just say you had to replace some parts because the old computer broke down or something on those lines. I've used the same copy in three builds, never have had to call. Only had to call once with XP

A OEM copy should work, I've done it before. You can use the phone activation method, you dont need to talk to anyone. Its a Automated process that asks you for a bunch of numbers and reads some back to you afterwards to reactivate windows. Easy.

i know i shouldend post it but, look some torrent sites, there are enough windows 7x64 ultimate iso´s totaly aktivated  like hunter editions, eaven windows 8 pro  iso x64 :P then you have youre os for free. and unattended installs

please dont spank me :p