Question about computers?

I have a computer with windows vista. It got infected with viruses so I reinstalled windows vista. I also have an external memory, it's not an external hard-drive, it's like a stick, but with 300 GB. It worked perfectly fine before I formatted the computer and reinstalled windows, but now, after I reinstalled windows and connected my memory to the computer, it said it's inaccessible and needs to be formatted before it can be used. I don't want to format it, I have important stuff on it. Is there any way to make it work( be able to access the stuff there ) without formatting?