Outlook Web Access is installed by default with every Exchange 2000 installation. So, unless you have done something non-standard, like uninstalling IIS or removing the OWA folders from IIS, it is already installed and running. Permissions is a bit tricky with OWA. I've had some installations where everything magically got configured correctly right from the get-go, but usually not. Here are a few MS knowledgebase articles that might be helpful:
http://support.microsoft.com/default.aspx?scid=kb;en-us;322309
http://support.microsoft.com/default.aspx?scid=kb;en-us;292723
http://support.microsoft.com/default.aspx?scid=kb;en-us;282964
(be sure to remove any spaces in these urls)
Hope this helps!