Trouble logging in to web server

I recently installed NIS 2010 on two home computers, one running Win Vista Home Premium and the other running WinXP.  I can now no longer log on to the web server that hosts two of my websites.  The webhost online software is called WebsiteOS.  On the login page, I enter my domain name and password.  After clicking Login, my account page begins to load, quits, and kicks me back to the login page.  This happens on both of my computers, on both website accounts.  I’ve tried using Firefox 3.5.x, Internet Explorer 8, and Google Chrome.  None will get me into the website.  I have contacted the tech support people for Website OS (24hourwebhostingsupport.com) as well as the local tech support person who handles our webhosting account.  None of them has trouble getting in to my website using my login information and say that the problem has to be at my end.  I do have a PDA running Windows Mobile 6–with no Norton protection–and it will log in properly, although the display is way too small to be used practically.  I’ve tried bypassing the router and connecting directly to my satellite modem; no joy.  I’ve disabled the Norton Smart Firewall and the Norton Anvirus Auto-protect; again no joy.  I’ve tried clearing the browser cache and cookies; alas, no joy.  I’ve rebooted the computer (cold boot); you guessed it, no joy.  Can anyone offer a new suggestion that might help?  I am not proficient at the NIS settings (preferring to “set it and forget it”), so I don’t know where to begin there.  Would appreciate ANY assistance!  TIA.