30 is actually the usual number but 29-31 happens as well. I've looked at task manager on the number of open windows and just ctrl+n:ed a folder repeatedly, it's always within 29-31(I think I managed 32 once).Hmm. I imagine you're not being literal about the number, but can you reproduce on demand? I.e. if you go to CWindows, select n folders, right click 'open in new window', then select one more and do the same, it fails at same point every time? If it is that simple to reproduce should make diagnosing it easier. Or at least cause it to happen at approximately the same number of windows.
One lazy thing to try.
View->Options->View->'Launch folder windows in seperate process'
If that fixes it without downsides memory wise.. I'd just take the win. Otherwise, start in safe mode. See if can reproduce. If not, check Autoruns (Microsoft Sysinternals) for Windows Explorer shell extensions etc. Disable, restart, attempt to reproduce, look for problems. If not something like that I'd suggest looking at what antivirus/antimalware software you have installed.
I run explorer in a separate process and I've unchecked that and tried, same result.
Thumbnails and everything like that is turned off both in folder options and advanced system settings, so it's not that, I even checked the thumbcache and it sits at 0kb. It doesn't actually matter what kind of folder it is, I've repeatedly opened "my computer" and it still hangs or starts drawing wonky windows aaand I just got the idea to shut off every graphical feature(this text looks like ass now) and try that.
Nope, 31st window is a faulty window that hangs. Tried it again with everything turned on, 30th window hangs. If I try again it might be the 29th, 30th or 31st, that's the range.
The faulty drawing of the GUI sticks around until a window is minimized/maximized or anything like that that forces windows to redraw it in it's entirety(windows doesn't redraw sections that it thinks is unchanged, so the wonkiness sticks around in the framebuffer), so Windows itself doesn't seem concerned.
It's not a graphics driver issue though.
There's no change in amount of RAM used before or after that, same with graphics memory and disk activity.
The only lead I have, or suspicion, is that ~30 is around the time when the list of open windows would get that crappy scroll button.