It takes a lot of work to get Windows 9x working on DOSBox from scratch.

The "could not extract geometry" error is most likely because the setting for the hard-drive image in the dosbox.conf file are not correct.

The DOSBox harddrive emulation has a fixed maximum amount of cylinders. As a result normal hardrive images can only be 512MB maximum. People that use bigger images use a hack by increasing the sector size. This gives them 2GB images, that is the maximum.
Also DOSBox only supports raw images, as far as I know (so 203MB for the uncompressed image would be incorrect, and indicate a corrupt or incompatible image).

A lot of good (but sadly sometimes slightly outdated) information on dosbox can be found in the vogons forum, including Windows9x guides:

http://vogons.zetafleet.com/viewforum.php?f=39