If all you did was move a folder, you won't ruin her computer (shoot, even if you did more, it won't.
There is only one gotcha.
Her computer harddrive drive is set up as a master; Your computer drive is set up as master. (both she and you have other drives as well).
To ensure all goes smoothly, remove your drive. Open her case. Find her harddrive. (hopefully only one if she has a standard system...if she has two, then she already knows how to hook two up).
Look at your drive. There should be a sticker on it telling you how to set it up as a master or slave, by switching a little piece of plastic known as a jumper. Switch yours to slave.
Hook up your harddrive to the same ribbon as her harddrive. If it is connected to a CD, you can remove it for now..Connect power to your drive. Turn her computer on.
The computer will boot up, find both drives. Her computer will be the C: drive. your's will be something else.
Open MY COMPUTER, move the WINDOWS folder back to the root, shut down, remove your drive, reconnect hers, reset the jumper so it is a master drive again, put your drive back.
Or, if you want to make it even simpler. Go to Compusa, buy a USB harddrive enclosure. Remove your drive, place it in enclosure (it should be set as master for USB enclosures), connect harddive enclosure to usb cable and plug it in to your girlfriends computer. Open explorer, do as above. Remove from enclosure, Put back in your tower.
An enclosure will cost about $20 to $30. (cheaper still on Newegg)