You'll need to make an image of your card, and you'll need at least as much free space on your computer as the capacity of the card.
Download Win32DiskImager, just Google it, it's free and you'll find plenty of sites to download it from. Unzip the binary file into a folder, all the files needed to run the program will be in that folder.
Now put your card into a card reader and insert/connect it to your Windows computer. Open an Explorer window and look for a drive named Boot, make note of the drive letter your computer assigns to it.
Start the Win32DiskImager program by double clicking on the Win32DiskImager.exe file. A small box will pop up, the box on the right labeled Device should have a drive letter in it and if you don't have any other external devices connected it should be the drive letter of the boot drive. If it's not use the dropdown arrow and select the correct drive letter for your boot drive.
To the left of the drive letter is another box with a folder to the right of it. Click the folder and browse to the location where you want to save the image file. In the file name field at the bottom type in what you wish to name the image file and click save.
This is important. you will see four buttons on the bottom, the only ones you need to concern yourself with are Read and Write. If you hover your mouse over the button you'll see a description of what each button does. You want to use the Read button "wite data on Device to Image file". Push the button and wait for it to finish.