Have you tried using another browser? On all other sites are you able to "surf" from page to page? Does this just happen on log in pages?
Do you have java enabled on IE7? On the other computer you have tried the login...is that computer running IE7?

Also have you done this?
Deleted all temporary internet files, including offline content (IE tools, Internet Options, General tab, Delete button). Delete your IE history and all cookies. Then, restart IE by clicking File, Exit. Then restart and see if you can access.