what you could try is to make a new user account give that user administrator rights and try to install with that account. Because it looks like it has something to do with user profiles.
The installation is on a local system with it's own hard drive. The system is networked wirelessly with 2 other computers, but I do not mean to use Media Portal with either of the others, only with the computer it is being installed upon. I do have the latest version of .NET installed, so that should not be the problem.