In this situation, you log into the site, but you get the error message "Access denied" (image). This is not the same as a message saying "Invalid username or password" (in that case, try again, or get a one-time login link sent to you so you can change your password.)
If you log in and get the "Access denied" message, it's almost certainly because your browser is not allowing cookies. Nearly all websites require cookies to work in order for visitors to log in. Google has an article explaining how to enable cookies on most web browsers here.
If you can't resolve this, you may want to use a different web browser, and install Chrome or Firefox. If you can't install Firefox because you don't have permissions on the computer, try Firefox Portable Edition, which doesn't require any installation.
Any program that blocks cookies on your computer or your network can cause this problem. For example, CA Security Center may block cookies in all programs. Try temporarily disabling your firewall and virus protection to see if they're the problem.