Hi.
Now, you can download a trial version of Ruby/Tk-Kit for RubyInstaller
1.9.1p378rc2 (tested on Windows XP). Ruby/Tk-Kit version of tcltklib.so 
includes full environment of Tcl/Tk. So, you don't need to install Tcl/Tk 
on your windows box.

You can download at 
http://www.dumbo.ai.kyutech.ac.jp/~nagai/RubyTk/?Ruby%2FTk-Kit
(The page is written in Japanese. sorry.)

RubyInstaller 1.9.1p378rc2 doesn't include Ruby/Tk files. At first,
please download "rubytk-simple-w-img_ext-for-RubyInstaller191.zip".
The file is a full package. Please extract it at RubyInstaller's 
lib/ruby/1.9.1 directory.

"tcltklib.so-simple-20100520-ruby191-mingw-for-RubyInstaller191"
and 
"tcltklib.so-w-exts-20100520-ruby191-mingw-for-RubyInstaller191"
are files for replacement of tcltklib.so.
The former includes standard Tcl/Tk only, 
and the latter includes many Tcl/Tk extensions.
If you want to use the one of those, please rename it to tcltklib.so
and copy to RubyInstaller's lib/ruby/1.9.1/i386-mingw32 directory.

The structure of tcltklib.so is same as Tclkit for Tcl/Tk.
It means that the tool "SDX" (http://www.equi4.com/starkit/sdx.html) 
for Tclkit (Starkit) can treat tcltklib.so of Ruby/Tk-Kit.

If you try it, it's my pleasure.
Any kind of reports are welcome.
-- 
Hidetoshi NAGAI (nagai / ai.kyutech.ac.jp)