you can do the extraction in 2 different methods and both of which uses the dumptool.exe program provided by the wiikey 1.2 configuration disc.
Option 1
Run the dumptool.exe using your windows xp console command, described above, and use the command dumptool i -sd (taking into account 'i' stands for your sd card drive letter) and it will extract the data game chunk from your sd card and create(and append it) an iso file automatically. Keep doing this for every chunk until all the game has been dumped.
Option 2
Run the dumptool.exe program as per normal but instead of using the -sd parameter you would use the -d option and have the command dumptool i -d which will extract the current game chunk stored in your SD card and saves it as an *.rvl file. Keep using this method for extracting all your game chunks and once you've acquired all chunks, use the copy command under the command console
copy /b file1.rvl+file2.rvl+file2.rvl game.iso to join them together to form one final iso wii game file.
Note- file1, file2 and file3 would be the name of the game chunks and game.iso would be the name of the iso file you want it to create once all files are joined.