The login URL for ASP.NET applications (including MVC3 ones) is controlled in web.config, in the forms authentication section:
<configuration>
<system.web>
<authentication mode="Forms">
<forms loginUrl="~/Home/Login" timeout="2880" />
</authentication>
</system.web>
</configuration>
The trick for you is that you want two different login URLs. ASP.NET has a great feature where you can have a web.config file in each directory of your project, and as needed it will use the most specific setting it can find, up to the root web.config. So in the folder where you have your admin views ("Admin" I'm guessing), you should be able to create a second web.config, which will apply only to those pages and lower in the tree:
<configuration>
<system.web>
<authentication mode="Forms">
<forms loginUrl="~/Admin/Login" timeout="2880" />
</authentication>
</system.web>
</configuration>