Thanks for the additional details gpfsmurf.
My current theory wrt this is that the problem is caused by one or more explorer shell extensions, which get loaded when the open/save dialogs are opened, as well as when directories are changed and tooltips shown within the dialogs.
There are documented cases of Acrobat Reader 7 causing these problems, which can be fixed by the host application calling CoInitialiseEx. Initialising COM is the change in 20080623 that I had hoped would fix this, but it seems not to help. Furthermore, Lorenzo tried uninstalling Acrobat, which didn't help.
I've been unable to reproduce this issue on any machine, which again points more towards an interaction between other installed software (presumably in the form of explorer shell extensions) and the open/save dialogs.
It'd be great if you could take a look at ShellExView (nirsoft.net/utils/shexview.html), and send me the list of shell extensions you have installed: It may lead me towards being able to reproduce the problem myself.