AFAIK, there's no easy way to remove etag of web resources such as image, css or js files from IIS 5. As each of the etag value regenerates on each subsequent web browser refresh which is unnecessary since there might not be any changes on those web resources.
You might want to try EtagFix, it doesn't remove the etag though but it stabilizes the etag values so they don't change until the web resources are modified.
EtagFix stabilizes the etag that
Internet Information Services (IIS)
generates so that it doesn't change
each time the web server is restarted.
http://www.isapilabs.com/Products/ETagFix/index.htm