XTerm won't start on Win2000

[Susannah Fleming]

I'm a newbie to Cygwin X, and I can't open an XTerm.  I've tried various 
options including startxwin.bat, startxwin.sh (from a Cygwin bash shell) 
and manually typing in commands from startxwin.sh.
When running the bat file in dos, I get nothing - no error messages and no 
xterms.
 From bash, I still don't get xterms but I do get an error message: "xterm 
Xt error: Can't open display: 127.0.0.1:0.0"
I've tried reinstalling the whole of my Cygwin dist, just Xfree, fonts and 
zlib with absolutely no effect.
I'm not using ssh and I have a colleague who should have an identical PC 
build who uses Xfree fine, so I know it's possible.
There is no X log being created in /tmp.  In fact, /tmp is empty.  Having 
read other posts, I find this distinctly worrying.
I'm running Win2000Professional and I've checked for suppressed pop-ups 
about missing dlls.  There aren't any.

Sorry not to provide more information but this is all I have!  Any clues 
for where to find more logs, or (even better) a solution to my predicament 
would be greatly appreciated.

Susannah