Your friend might need to buy another windows license, as you may only install windows on one computer using one license. As Virtualbox behaves as a seperate computer, you'll need a unused license(or you can pirate it, but I don't think "that kind of information" may be shared on the pclinuxos forum...).
Do be aware that with most manfacturers, you lose your guarantee when installing another operating system. If your friend has two hard disks, I would recommend installing pclos on one, and then take the hard disk out if you want to repair it with guarantee.
But that's just an option. If your friend doesn't need guarantee, and is willing to pay the repair costs(if any), it would be best to backup all of the important data on the computer. Then make the windows partition smaller, and install PClinuxOS next to it(just to be sure, so if it doesn't work out the way you want, you still have your windows installation). Then install virtualbox from the repos, and make sure your friend has got a windows cd + unused serial key. Install windows in the virtualbox, and make sure it works.
At the end, you can remove your windows partition. Or, if you used two hard drives, you can keep it...