Just when I think I customized my last OWA page, I get another request. Usually it’s to add a logo or take away the “Domain” part of the logon form. Anyway, it’s something that I don’t do often but enough for me to write this. This is more of a reference for me so I can find the instructions when I get the request again and maybe it might help someone else. Also, be sure to backup the logon.asp file. If you mess up that file, OWA won’t come up. I did that once and I had to reinstall sp2 to get the file back. I only made that mistake once.
http://www.msexchange.org/tutorials/Customizing-OWA-2003-Forms-Based-Authentication-Logon.html