The windows version of sdminst is contained in this zip file ->
http://stereo.jpn.org/eng/sdm/file/common_files.zipMake sure you extract it to a separate directory (because there might be files that conflict with your current CHDK). Then run "sdminste.exe".
Since you are using CHDK, ignore the "auto" section, as well as "Copy SDM to SD Card" in sdminst.
You will want to focus on the "create partition", "format" and "Make bootable disk" buttons. MAKE SURE that the proper SD card/disk drive is selected.
When you're done, copy your CHDK files to BOTH partitions. It's just easier that way.
Good Luck!