Try this :
<%= Html.ActionLink("Back", "Index", "Home", new { area = "" }, null) %>
When using Areas, you should always specify the area your are calling in your ActionLinks by adding a route value as above, If the link is outside the area (as in your case), just use an empty parameter for the area.
There’s a nice extension that i find essential in any ASP.NET MVC project (T4MVC). It makes your ActionLinks look much cleaner and it protects them against errors.
So the above code will look something like this :
<%= Html.ActionLink("Back", MVC.Home.Index()) %>
and when using an area :
<%= Html.ActionLink("Some Link", MVC.Admin.SomeController.SomeAction()) %>
It’s a part of the MvcContrib project on codeplex here
You should consider using it.