Hello Nick, 

> D:\RMagick-1.9.1-IM-6.2.3-win32>gem install
> RMagick-win32-1.9.1-mswin32.gem Attempting local installation of
> 'RMagick-win32-1.9.1-mswin32.gem' ERROR:  Error installing gem
> RMagick-win32-1.9.1-mswin32.gem[.gem]: buffer error 
I can reproduce that on some machines, although not on my testing machines. 
This is an error that somehow floats up trough the zlib library into 
rubygems, there is even code in rubygems to avoid that error, but it seems 
that it has hit again. (search for the words 'buffer' or 'error' in 
package.rb from rubygems)

I cannot really say how to fix this, I tried for a whole afternoon to no 
success. I guess replacing zlib.so would be an alternative, but I can't see 
where to download one. You could also try to enable the error avoidance 
branch in the above piece of code, maybe that solves the problem ? 

If things get really tough, I can whip together a manual install, although 
that again would be a lot of work. 

Tell me what works for you
kaspar

-- 
code manufacture & ruby lab at http://www.tua.ch/ruby