This is XEmacs 21.4 (patch 3) "Academic Rigor" [Lucid] (i586-pc-win32).
I just got a customize menu freeze (xemacs completely hung) by choosing
Customize->Emacs->Programming->Languages->Sgml
As soon as xemacs starts to build the Sgml menu, the sgml package prompts
for your email address in the minibuffer. At this point, xemacs is
completely frozen, and has to be manually killed. I've included a
screenshot.
No idea if this is win32 only, or not.
-Jim Potts