If you are using Vista you don't have to reformat, you can use SYSPREP to transfer you Windows installation to the new system.
Here are instructions on using SYSPREP.
Originally Posted by Trotter
You can use SYSPREP to prepare your hard drive for transfer and not have to reinstall anything. Use SYSPREP just before
you are going to move your stuff over to its new home.
To use the GUI (graphical user interface), copy and paste"c:\windows\system32\sysprep\sysprep.exe" in the Start Search
box. You should get something like this:
Change the Shutdown Option to Shutdown so it doesn't reboot on you. Once it shuts down, it is ready to move. When you boot it up in the new hardware, Vista will act like it has just been started up in your computer for the first time. Once you are booted you will have to install the drivers for your new hardware.