[Short recount of the problem]
Accessing efs like filenames like /C: is currently impossible.
The solution adopted by modern FSF emacs and efs versions is to use
the /: syntax. I.e. access /:/C: instead.
In fact that syntax even works for the efs version that corresponds to 
XEmacs 21, in the sense that efs now uses /[^:]+: as the regexp so efs 
won't touch it. Unfortunately the core of XEmacs does not know how to
turn "/:path" into "path". That is probably a bug.
A perverse work around is to use /localhost:/C:
Jan
-- 
Jan Vroonhof                    
http://www.math.ethz.ch/~vroonhof/
Mathematik,                               vroonhof @  math.ethz.ch
HG E16, ETH-Zentrum,                      Tel: +41-1-6325456/25154
Raemistrasse 101, CH-8092 Zuerich.              Fax: +41-1-6321085