SIRS login prompt after logging in

Why do I get prompted to log on to SIRS after I've already logged in at this computer?

When you log on to SIRS, a cookie is placed on your hard drive. If this cookie cannot be placed on your hard drive, or gets erased, you will be asked to log on again. If you continually receive this prompt, make sure your browser is configured to accept cookies and you disable any security software while logging in.