I've been looking around the forums and a lot of people have been having problems with the 2.1.0 installation of VirtualBox, and I have found a solution. Now, probably a lot of people have already found this, and I acknowledge this. Now, onto the tutorial
Pre-installation
I do not know why the installation ends prematurely. Here's how I got around it. You're gonna want to download VirtualBox 2.0.6 here. Then, run the .msi that you download. IMPORTANT!!! You MUST make sure no previous version of VirtualBox are installed. (Updating through the client will not work. It will just say you have the latest version.) So, install 2.0.6, and go to the next step.
2.1.0 Installation
Download the 2.1.0 installation file for VirtualBox. Run it over top of your previous 2.0.6 installation. You should get the "This has not passed the Windows Logo Test". Press Continue Anyways all three times. If three windows do not pop up, and only 2 do, it might not install. If all goes well, the installation should finish and VirtualBox should start normally.
2.1.0 Screenshots
Other
If you have any questions, please feel free to ask.
If this has already been posted somewhere, please inform me.