first, you need to have a license. if the copy of windows you have right now was isntalled already on the first PC when you bought it, then you can only use it for THAT pc. a new PC requires you to buy a new license.
now if you bought a full license before, its transferable. but you would still need to re-install it onto the new PC, otherwise all the drivers would no longer match up. the software on the HDD is for the PC it was in originally, not the new components.
sounds like you need to buy a new windows license. or if you have a transferable license and a disk, you can use the old HDD and reinstall windows.
It amazes me that people will spend hundreds and even thousands of dollars on top of the line hardware and then quible about having to pay for the operating system. Some will pay $500 for a video card and complain about $200 for the OS.