I'm assuming that you are new to DotNetNuke, so let me suggest that you begin by viewing some of Lee's tutorials. In particular, I'd recommend:
An Introduction to the Principles of DotNetNuke
Quick Start Guide to DotNetNuke How to install DotNetNuke 4.5 to Windows XP Pro with IIS
How to install DotNetNuke 4.5 to Windows XP Home (Using VWD)Now, running DotNetNuke inside of Visual Studio is a complication that you don't need at this point. The good news is that you don't have to reinstall anything. Using the material in the tutorials, what you do need to do is create a virtual directory using IIS and point it at the directory that contains your installation of DotNetNuke.
Once you have done that, you'll be able to go to
http://localhost and see the first portal. If you can see the first portal, it's time to move on to the second portal.
You didn't say how you created the second portal. I would recommend that you make it a child portal, so that you won't have to get involved in fiddling with URLs right off the bat.
You should be able to login as superuser to the first portal. When you do, go to the host settings, then go to the portals item and edit the portalalias for each portal and remove any changes or additions that you made. Then go to the second portal and make it's type a child.
I'll assume that you've named the second portal Portal2. It's URL will then be
http://localhost/Portal2. You should be able to browse to that portal.
Let us know if that helps. If not, tell us what happended and I'll give you some more suggestions.