You have been able to run groupwise server on win32 since at least the gw5.5
days, maybe even 5.2. You can get specific info from
news://support-forums.novell.com (has some nice gw6x forums) & of course
http://www.novell.com/documentation.
For one client we ran gw55ep on nt4svr smoothly.
btw, come brainshare next yr, they will release a version of groupwise that
can run completely on linux. Now thats what a call crossplatform support,
full support for win/lin/nw, & some of the agents like webaccess you can run
on anything that can run apache (or any other webserver pretty much) like
solaris.