> irb(main):001:0> require 'tk'
> LoadError: tcltklib: tcltk_stubs init error(-1)
>          from /cygdrive/c/RUBY/lib/ruby/1.6/tk.rb:7:in `require'
>          from /cygdrive/c/RUBY/lib/ruby/1.6/tk.rb:7
>          from (irb):1:in `require'
>          from (irb):1
>
> C:\WINDOWS>ruby --version
> ruby 1.6.5 (2001-09-19) [i386-cygwin]

This is my least favorite response, but, "it works for me". I just did a
clean install of "ruby165-2.exe" from the Pragmatic Programmers' site on my
Windows 2000 box, then repeated your experiment as shown and the "require"
returned "true".

Can you check your environment settings to confirm that the installer
correctly set the values of RUBY_TCL_DLL and RUBY_TK_DLL?

    C:\>echo %RUBY_TCL_DLL%
    C:\ruby\tcl\bin\tcl83.dll
    C:\>echo %RUBY_TK_DLL%
    C:\ruby\tcl\bin\tk83.dll

and of course confirm that those two DLLs are actually sitting there?

Lyle