The problem is in Windows and since I'm unable to fix Microsoft's code it's unlikely that there will be a fix any time soon. As a workaround you can try to disable the native transparency in Rainlendar or the Aero desktop in Vista to see if that helps. Or just use some other position than On Desktop.

I'm not using the On Desktop function, actually.

When my system starts up from Hibernation, the Events window is always in the upper-right quadrant of the desktop. (Not the specific part of the corner that I set it at.) Sometimes I can move it, other times it behaves like I'm painting in Photoshop. (No grabbing possible.)

Today I found another instance of all programs appearing under the calendar, instead of over it. Even the Rainlendar context menus appeared under the calendar, instead of over it. Odd.

If the development of Rainlendar will take some time, I'll investigate using the Vista sidebar and calendar gadget. It's not as useful, nor does it have all my dates configured.

If you disable the native transparency (Options->Advanced->Transparency) does it fix the problem?