With my camera if i transfer them onto my computer they still stay on my camera to. not sure about yours though. maybe try looking on the left hand side of your 'My Pictures' and see if there is something there. i really have no idea because i don't know what camera you have or what memory stick. sorry that probably did not help at all.....
You should be able to copy them right from your computer back onto your memory card IF you do not plug your camera into the Computer. So in other words if you use a card reader your computer will read your card just like it was another memory storage unit and you should be able to copy right back to it. DO NOT CHANGE the extension on the photos I assume they were saved to the memory card as a jpeg and you want to save them back in the same format.
If you are having problems finding the photos on your computer typically your photos are saved into the folder labelled "My Picture" however you may have subfolders in there and you will just have to root around until you find them.
You could always order prints on line from home and have them ready when you go and pick them up. Actually right now I think Walgreens has a special on line wher if you order 25 prints you get 25 free check out there web site, I think it goes on till the 10th.