If you replace major components (MoBo, HDD) you may have to contact Microsoft to get a key (its long - 20 digits or so) to re-activate Windows.
As long as you have a registered copy of Windows and you have not called tech support for a system change prior to this occasion you should be able to get the key.
I went through this 6 months ago when I did a major overhaul on my system.
You will need to have your original installation CD to install the OS on your new (used) HDD.
I don't believe that copying and pasting will do the trick. If I recall accurately, I attempted something along that line and it failed. OLE (Object Linking and Embedding) technology does not work for items such as ISO files and OS files.
In the end, I spent 30 minutes on the telephone with a tech in India (I think). It wasn't too bad of an experience.
[SIZE=\"1\"]E6400 @ 3.4 , Noctua 120
7900GT @ 670/1680 , VF900
2 x 1GB Patriot 6400 @ 425
WD SATA 250GB
Neo HE 550
P180 / G15 / MX1000[/SIZE]