You can't transfer the operating system. If you want to install XP on the desktop, you'll need an installation disk and a COA number (does the desktop have a sticker with this info?). Installation is straightforward, but you may need to phone MS to activate the installation as the mobo has been changed. Transferring data is simple with an external hard drive to copy from the lappy, then transfer to the desktop. Be prepared to spend some time finding drivers for the hardware on the PC.
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