Plug the 200GB drive in and transfer via copy and paste, under WinXP it will automatically prompt you to format the drive if you haven't done so already. As far as I understand a "proper" mirror would necessitate you buying two identical drives and setting them up in RAID-1.
You will need some third party software to do this, the hard drive makers website will have a utillity for cloning the data from one drive to another
The best software out there as far as I am concernerd is Acronis True Image.
It will clone your old drive over to your new drive
It will restore from backups and much more.
Very user friendy to use
BTW copy and paste is not going to transfer your OS
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