After that you need to install the Watir gem. If you are installing behind a proxy, then you first have to set the HTTP_PROXY environment variable:
set HTTP_PROXY=http://username:password@your.proxy.com:80
After that you just need to run following command to install Watir:
gem install watir
This will install drivers for both IE and Firefox. Firefox also requires a jssh plugin for Watir. Links for installing the plugin can be found from http://watir.com/installation.
If you also want to run tests against Google Chrome. Then you also need to install the watir-webdriver gem:
gem install watir-webdriver
Now just start to write and run your Watir test scripts :)
If you get some complains about not finding Watir when trying to run your script. Try to set following environment variable:
set RUBYOPT="rubygems"
Some more information about the RUBYOPT parameter can e.g. be found from here.
NOTE! That you should run all the above commands with Administrator privileges.