While installing the newest version of Oracle's Virtualbox on my Windows XP machine at work, the progress bar stopped just short of the end. And there it stayed ne'er to move again. It had taken down my LAN connection and no Repair or other magic would revive it.
I struggled to wrap my head around this. I killed the installation process and rebooted the computer. When I tried to start Virtualbox again, I received an error regarding my network card. Finding no Virtualbox installation candidate in the Add/Remove Programs menu*, I simply tried to start the installation again. I received warnings about needing to rollback changes from a previous installation. Yet again the progress bar froze.
My LAN connection was down again, so I ran a Google Search and found this post from Keith Elder's blog suggesting that the Dropbox client was the culprit. I closed Dropbox and, immediately, the progress bar jumped into action and I was able to complete the installation.
* Admittedly, I didn't look very hard, but nothing under V for Virtualbox nor O for Oracle led me to believe it wasn't going to be there.