Whether or not the hard drive has Windows installed on it is irrelevant to the actual process of replacing the drive. if you want to preserve the current Windows installation, you'll need to use hard drive imaging software to copy the data from the old drive. Otherwise, you can simply replace the drive and reinstall Windows as you would normally.
You do not have to reinstall Windows if you wish to add an additional drive. If you want to be able to boot from that drive, or to replace your current boot drive, you will need to install Windows on it.
A no disk error is the result of the hard drive crashing. Replace the hard drive, and reinstall the proper Windows software for the specific computer.
If you have not used disk imaging / cloning software to transfer it to the new drive, yes. You can still reinstall with the disc, however.
You would need to buy Windows XP and install it on your hard drive to replace Windows 7. However there are programs that can run a simulator of XP while running Windows 7.
The hard drive stores the Operating System. Like for example : Windows XP or Windows Vista or even Windows 7 without the hard drive your computer will not turn on.
Windows XP has no bearing on the hard drive size beyond what is needed to store it. Thus a hard drive on which Windows XP is installed can be anywhere from 1.5 GB to 3 TB.
Yes you can have the Mac OS on one drive and Windows on another drive. Or you can partition a single hard drive and have both on the same drive.
Yes
You can certainly move Windows to an external hard drive but Windows will not boot directly from an external drive. If you are running Windows in Parallels (See links below) you can have Parallels installed on the Mac's drive and then have your Windows virtual machine on the external drive.
During installation, Windows 7 will need about 16GB on a hard drive.
no. formatting a hard drive means completely removing all files, including windows...
I'm not quite sure what you mean, so I'll give you both answers: "How do you use a hard drive for Mac and Windows on the same machine?" On a mac, insert a Windows install CD or DVD, and the computer will ask you if you want to set up windows to run alongside mac. In the process you will partition the hard drive, so you will kind of have a windows hard drive, and a mac hard drive, but on the same disk. "How do you remove a Mac hard drive and put it in a Windows machine?" I am not a technical expert, so you might want to look up "How to remove and install a hard drive". All I know is it probably won't fit, so you might need to buy a hard drive enclosure and run the drive from a USB port. Hope one of these helps! If not, please revise your question