I am having the same problem:

C:\dev\Utility\scripts\ruby>ruby login.rb
c:/apps/ruby/lib/ruby/gems/1.8/gems/watir-1.5.4/./watir/frame.rb:32:
[BUG] Segme
ntation fault
ruby 1.8.6 (2007-09-24) [i386-mswin32]

This application has requested the Runtime to terminate it in an
unusual way.
Please contact the application's support team for more information.

------------
here is my (simple) script that i'm running on xp sp2, ie7:

require "watir"

testsite = "https://my.site.com/"
ie = Watir::IE.new
ie.goto(testsite)
ie.text_field(:name, "UserName").set "myname"
ie.text_field(:name, "Password").set "mypassword"
ie.text_field(:name, "Client").set "myclient"
ie.button(:id, "Submit").click
ie.wait
expected_url = testsite + "Home/UserHome.asp"
if ie.url == expected_url
       ie.span(:text, "View/Modify Items").click
       ie.wait
       ie.radio(:id, "Filter").click
       ie.wait
       ie.select_list(:name, "Filter").click
       ie.select_list(:name, "Filter").select("saved filter")
       ie.wait
       ie.button(:id, "Submit").click
end

this works on xp sp2 machines that are running ie6, oddly enuff...