I'm not completely sure, but this may be indicative of either a hardware or a drivers issue -- possibly drivers.
The W7 drivers model appears to be somewhat different to the Vista ; in my experience anyway, W7 7100 includes some beta drivers that do not always work exactly as desired. On my own main computer, it installed a bad ethernet driver which would always cause connection issues and would often cause hang and even BIOS instability, problems which seem to have vanished after a new driver appeared on WU and was automatically installed on my rig.
Of course, the above symptoms could also occur upon some hardware failure -- BUT the W7 and Vista drivers models are NOT so dissimilar that hardware-related issues in W7 would not be replicated in Vista, in my opinion.
Of course number 2, it could also just be an old bug that has since been fixed.
I would check WU for the optional updates, and see if any new hardware drivers are available. You could also scour the websites of the manufacturers of your hardware components for new W7 drivers, including for your graphics card.
Anyway -- is it just Explorer freezing specifically, or do all open windows on your desktop hang simultaneously ?
It is possible though, that only patience and/or W7 RTM installation would solve that issue.
I feel that it's far more likely to be a beta drivers issue than anything else