What is the best technique to keep my Winsows XP from my old PC when getting a new PC ?
I own and recommend a copy of Acronis True Image. It allows you to create a complete image of your hard drive and restore it to another new drive very easily.
When restoring it to a new PC with a different motherboard, video card etc, upon restarting Windows will complain and sometimes blue screen.
To get around this you can boot from the Windows XP CD and choose to "Repair the installation". This will update / replace drivers with correct ones for the new hardware.
Alternatively, Acronis also offer a "Universal Restore" that is an add-on that automatically ensures your image is compatible with your new hardware.
I don't work for Acronis and have nothing to do with them apart from being a customer who is generally pleased with Acronis TrueImage (I still use version 8 and have no reason to upgrade).
I actually think it's not easily done: Windows installations are very dependent on the hardware, so unless your new PC has exactly the same hardware as the current one, you're out of luck.
(I think this even includes the Windows activation, which is somehow bound to the hardware it was first installed on, and only allows for minor configuration changes without contacting Microsoft support.)