On the MedHelp home page, there is a box that can be "checked" (X) to remember your details for next time you log in. That works for me.
If you have your browser set to deny cookies or delete them at the end of a browsing session, then I think you'll still have to log in whenever you return to MedHelp. Usually there's a way to allow cookies from trusted sites while deleting others - someone in the Computer Users Group for should be able to answer that (
http://www.medhelp.org/forums/Computer/show/406).
Otherwise, there should be a "Login" link at the top of the various pages here, on the top right side of the screen just under the blue MedHelp banner.