Thanks for the link. That is the same issue I'm having.
Here is a script that will re-register the local installed Virtualbox Service on XP machines. I'm not sure if it will work on a Win 7 box.
Run it after you shutdown your Portable-Virtualbox, and that should reset the local installed version.
Copy and save as a .bat file. (Re-Register-Virtualbox.bat)
@echo off
CD C:\
CD C:\Program Files\Oracle\VirtualBox\
VBoxSVC.exe /ReRegServer
regsvr32.exe VBoxC.dll
EXIT
This post has been edited 2 times, last edit by "jkemp" (Feb 28th 2011, 8:58am)