"It seems it is a special hidden application window created by dot net (".net"
. When a program is finished with a control (OLE, ActiveX - things like check boxes, dropdown boxes, etc) in dot net, it can dynamically release it back to the system. But with dot net, it goes to this hidden application first. Why is it getting jammed when I reboot all of a sudden? I'm not sure. The most likely reason is it is in an application that I've installed or updated recently. Possibly it is BHODemon or even more likely it is the updated version of Norton Anti-Virus. NAV is showing up with some new icons during updates just lately, so that makes me give that a high likelihood. That and the fact that you would think NAV should be an early-in, late-out driver."
So in english, and for you:
The window has been created by an application that uses the .NET framework.
Presumably this would be one of the ones you mentioned installing:
I've installed Rainbar, Vista Hardrive and Winflip recently,
And since you said you mentioned removing Rainbar from the start up applications but it is still starting up, I presume it may well be this. Perhaps when it is starting up it is releasing a control of some kind, and therefore the parking window comes up, as this window recieves the control before removing it.
So most liely it is Rainbar, though since this doesn't happen to everyone we can presume you are just unlucky 
If it keeps happening after rebooting, then I would suggest uninstalling and reinstalling Rainbar to see if that fixes the problem.
It isn't app specific, it can quite literally happen with any app, but in your case it appears when windows is starting, so it is reduced to a start-up application, and a recent one.
EDIT: And after some more google searching this would appear to be not only because it is a control, but because of a program that isn't written correctly being held in memory, or part of it in memory, which brings up and holds the WindowsFormsParkingWindow...
I would just suggest you reinstalled Rainbar and see if that solves your problem, trusting rainbar is the problem, and it isn't another startup application.
You could just keep closing it but for two reasons:
1) That would get tiresome,
2) If there is an error in an application it is best to get rid of it, as it could be more dangerous than you think.
PS: Dont't go into remove/programs/updates/services/etc and simply uninstall the .NET framework. This would stop the window coming up but could also cause several applications to stop functioning completely, or worse function incorrectly.
